Cost of Goods Sold and Stock Valuation for Ecommerce Sellers UK: A Guide to FIFO, Weighted Average and Year End Stock Counts for Amazon, Shopify, eBay, Etsy and TikTok Shop Sellers

If you sell on Amazon, Shopify, eBay, Etsy or TikTok Shop, cost of goods sold for ecommerce sellers is one of the most misunderstood numbers in the entire business. Most sellers know their sales figure down to the penny because the marketplace shows it to them every day. Far fewer know what their stock actually cost them once landed costs, packaging, FBA fees and shrinkage are taken into account, and that gap is exactly where profit quietly disappears.

Cost of goods sold for ecommerce sellers is not just a line on a spreadsheet. It drives your gross profit margin, your corporation tax bill, your VAT position and, ultimately, whether the business you are building is actually making money or simply moving it around. Get your stock valuation wrong and you can end up paying tax on profit you never made, or worse, underpaying tax on profit HMRC will later ask you to explain. If you have not yet read our guide to ecommerce bookkeeping, it is worth reading first, since everything below assumes your day to day books are accurate enough to value stock from.

This guide walks through how cost of goods sold for ecommerce sellers works in practice, from the formula itself and the stock valuation methods HMRC accepts, through to landed costs, year end stock counts, multichannel tracking and the mistakes we see Amazon, Shopify, eBay, Etsy and TikTok Shop sellers make most often.

Why Cost of Goods Sold Matters for Amazon, Shopify, eBay, Etsy and TikTok Shop Sellers

Cost of goods sold for ecommerce sellers, often shortened to COGS, is the direct cost of the stock you have actually sold in a given period. It sits underneath your revenue on the profit and loss account and, once subtracted, gives you gross profit, the single most honest measure of whether your product range makes money before overheads, marketplace fees and staff costs are even considered.

Sellers who run their business from marketplace dashboards alone tend to watch revenue and bank balance only. Revenue can be growing month on month while gross profit quietly shrinks, because supplier prices have risen, freight costs have gone up, or a new product line has a much thinner margin than the rest of the catalogue. Without an accurate cost of goods sold for ecommerce sellers figure, none of that is visible until cash starts to feel tight, usually right when you need it most, such as heading into peak season.

Getting cost of goods sold for ecommerce sellers right also matters because HMRC expects it. Your stock valuation feeds directly into the figures reported on your self assessment return if you trade as a sole trader, or your year end accounts and corporation tax return if you trade through a limited company. A stock figure that has been guessed rather than calculated is one of the first things a compliance check will pick apart.

What Counts as Cost of Goods Sold for an Ecommerce Business

For a typical Amazon, Shopify, eBay, Etsy or TikTok Shop seller, cost of goods sold for ecommerce sellers usually includes the following.

• The purchase price you pay your supplier or manufacturer for the goods themselves.
• Inbound freight, shipping and courier costs to get stock from your supplier to you or to a fulfilment centre.
• Import duty and customs charges paid to bring stock into the UK.
• Packaging that becomes part of the product itself, such as branded boxes, inserts or labels included with every unit.
• Costs directly tied to preparing goods for sale, such as FBA prep and labelling charges.

What should usually sit outside cost of goods sold for ecommerce sellers, and instead be treated as an overhead or operating expense, includes marketplace referral fees, advertising spend, subscription costs for your accounting software, office costs, general storage fees not tied to a specific unit, and your own time.

Many sellers lump everything into one bucket, which flattens gross profit into net profit and makes it impossible to see which product lines are actually worth pushing. HMRC’s own guidance on allowable business expenses for the self employed confirms that stock and goods bought to sell on sit in their own category, separate from general running costs. If allowable business expenses in general are unfamiliar territory, our guide to tax planning and allowable expenses for ecommerce sellers covers the wider picture beyond stock.

How to Calculate Cost of Goods Sold: The Formula Every Ecommerce Seller Needs

The standard formula used across UK accounting, and the one HMRC expects to see reflected in your accounts, is straightforward.

Opening stock, plus purchases made during the period, minus closing stock, equals cost of goods sold for ecommerce sellers.

In practice that means three figures need to be right. Opening stock is simply last period’s closing stock carried forward, which is why an inaccurate valuation in one year quietly distorts every year that follows. Purchases is the total cost of stock bought during the period, taken from supplier invoices rather than bank statements alone, since payment timing and purchase timing rarely match exactly. Closing stock is the value of everything still unsold at the end of the period, whether it is sitting in your own storage, in an Amazon fulfilment centre, in transit, or with a third party logistics provider.

A seller who buys £120,000 of stock across the year, started with £20,000 of opening stock and ends the year holding £35,000 of unsold stock has a cost of goods sold for ecommerce sellers figure of £105,000, calculated as £20,000 plus £120,000 minus £35,000. That figure, not the £120,000 spent on purchases, is what belongs against revenue on the profit and loss account for the period.

Stock Valuation Methods Explained: FIFO, Weighted Average Cost and Specific Identification

Once you know what counts as stock cost, the next question is which unit cost to use when the same product has been bought at different prices over time, which is almost always the case once supplier prices move or exchange rates shift.

First in, first out, known as FIFO, assumes the oldest stock is sold first. It is the most commonly used method among UK ecommerce sellers and tends to reflect physical reality reasonably well for most product categories, since older stock genuinely does tend to move through fulfilment centres first.

Weighted average cost blends every purchase of a given product into a single average unit cost, recalculated each time new stock arrives. It suits sellers who buy the same product in frequent, smaller batches and do not want to track individual purchase batches separately, and it smooths out short term price fluctuations from suppliers.

Specific identification tracks the actual cost of each individual unit, typically by serial number or batch. This is common for higher value, lower volume goods, such as furniture, electronics or made to order items, where each unit genuinely has its own distinct cost.

HMRC accepts stock to be valued at the lower of cost or net realisable value, meaning if stock is damaged, obsolete or simply unsellable at the price you hoped, it should be written down rather than left at full cost.

HMRC’s own internal guidance on this is set out in its Business Income Manual on stock valuation bases. What HMRC does not accept for tax purposes is LIFO, meaning last in, first out, which remains common in some other countries but is not a recognised valuation basis for UK tax returns. Whichever method you choose, consistency matters far more than the choice itself. Switching methods between periods without good reason is one of the fastest ways to attract questions during a review of your accounts.

Landed Costs, Amazon FBA Fees and What Belongs in Your Stock Valuation

Landed cost is the true cost of a unit once it has actually arrived and is ready to sell, rather than the invoice price alone. For most Amazon, Shopify, eBay, Etsy and TikTok Shop sellers sourcing stock overseas, landed cost typically includes the supplier’s unit price, freight from factory to port and port to warehouse, import duty, and any inbound customs clearance charges.

Import VAT is usually recoverable rather than a stock cost in its own right, provided you are VAT registered and hold a valid C79 certificate or postponed VAT accounting statement, which is why keeping clean import records matters so much. Our guide to customs, import VAT and cross border tax for ecommerce sellers goes into this in more detail if you source or sell internationally.

Where Amazon sellers most often go wrong is treating every FBA related charge the same way. Costs to physically prepare and label a specific batch of stock before it becomes sellable are properly part of stock cost. General FBA storage fees, referral fees taken at the point of sale, and long term storage surcharges are operating costs, not stock cost, and should be expensed in the period they are incurred rather than absorbed into the value of unsold inventory. If you are funding larger stock orders ahead of a launch or a busy quarter, our guide to business funding and cash flow forecasting for ecommerce sellers covers how to plan for that without straining working capital.

Year End Stock Counts and Getting Your Closing Stock Figure Right

Your closing stock figure is only as reliable as the count behind it, and a physical or system based stock count at your year end is not optional if you want accurate accounts. For sellers using Amazon FBA, this means pulling an inventory ledger report as close to your year end date as possible, since stock sitting in multiple fulfilment centres, in transit between them, or awaiting inbound receipt can easily be missed or double counted if you rely on a single snapshot.

A reliable year end stock count should reconcile three things against each other, being the units your inventory management or accounting software shows, the units the marketplace itself reports as available or inbound, and where practical, a physical spot check of your own stored stock. Differences between these usually point to unrecorded damage, shrinkage, returns not yet processed back into stock, or timing differences around goods in transit.

Each of these needs a proper adjustment in your accounts rather than being ignored because the numbers are close enough. This is exactly the kind of detail our guide to year end accounts for ecommerce sellers walks through in the context of the wider year end process, and it is an area where statutory accounts prepared without a proper stock count are far more likely to need correcting later.

How Cost of Goods Sold Affects Your Profit Margin, Corporation Tax and VAT

Cost of goods sold for ecommerce sellers sits at the centre of your gross profit margin, calculated as revenue minus cost of goods sold for ecommerce sellers, divided by revenue. Two sellers with identical monthly revenue can have completely different businesses once this figure is worked out properly, and it is the number that should drive decisions about which products to keep, which to drop and which supplier price increases you can actually absorb. Our dedicated guide to profit margins for ecommerce sellers covers how to benchmark this against typical Amazon, Shopify and multichannel margins.

Cost of goods sold for ecommerce sellers also has a direct effect on your tax position. Overstate closing stock and you overstate profit, which means paying more corporation tax or income tax than you actually owe on real profit. Understate closing stock and profit looks lower than it really is, which understates tax due now and simply defers the problem, since it will correct itself the following year regardless of what you intended. Neither outcome is something HMRC treats kindly if it is picked up during a check, and both are entirely avoidable with a proper stock valuation process.

For VAT registered sellers, cost of goods sold for ecommerce sellers does not directly change your VAT liability under standard VAT accounting, since VAT is calculated on sales and reclaimed on purchases as they happen, not on stock movements. It does, however, affect cash flow planning around VAT payments, particularly if you are close to the £90,000 VAT registration threshold and stock heavy purchasing is masking how much of your revenue is genuinely taxable turnover. Our guide to VAT for ecommerce sellers covers registration thresholds and marketplace VAT rules in full, and our VAT service page explains how we support returns and registration for ecommerce clients.

Tracking Cost of Goods Sold Across Amazon, Shopify, eBay, Etsy and TikTok Shop

Selling across several marketplaces at once makes cost of goods sold for ecommerce sellers considerably harder to track, simply because sales data lives in five different dashboards while your stock is often shared across all of them. A unit sold on eBay this morning and a unit sold on TikTok Shop this afternoon might be drawn from exactly the same physical batch, and your accounting needs to reflect that shared pool rather than treating each channel as an isolated business.

This is where proper ecommerce accounting software earns its keep. Tools such as A2X and Link My Books can post sales, fees and refunds from each marketplace into Xero or QuickBooks accurately, but stock and cost of goods sold for ecommerce sellers still typically needs a connected inventory system or a disciplined manual process layered on top, since marketplace integration tools are built primarily for revenue and fee reconciliation rather than inventory costing. Our guide to accounting software for ecommerce sellers compares the main options if you are still reconciling manually across channels.

Common Cost of Goods Sold and Stock Valuation Mistakes Ecommerce Sellers Make

We see the same handful of mistakes repeatedly across Amazon, Shopify, eBay, Etsy and TikTok Shop sellers, and each one is straightforward to fix once spotted.

• Valuing stock at the selling price rather than the cost price, which inflates both stock value and reported profit.
• Leaving out freight, duty and prep costs entirely, so cost of goods sold for ecommerce sellers reflects only the supplier invoice.
• Never writing off damaged, lost or genuinely unsellable stock, so the balance sheet carries value that no longer exists.
• Treating an entire year’s stock purchases as cost of goods sold for ecommerce sellers, rather than adjusting for what is still unsold at the year end.
• Switching valuation methods from one year to the next without a clear reason or proper disclosure.
• Not reconciling marketplace inventory reports against the accounting records at all, so the two simply drift apart over time.

Several of these sit alongside wider bookkeeping habits that catch sellers out. Our broader guide to common ecommerce accounting mistakes covers the mistakes we see most often beyond stock specifically, and is worth reading alongside this guide.

How an Ecommerce Accountant Can Help with Cost of Goods Sold and Stock Valuation

A general practice accountant who mainly works with retail shops or service businesses will not always be familiar with the particular mess that multichannel ecommerce stock creates, from FBA inventory reports to marketplace return rates that vary wildly by category. A specialist ecommerce accountant who works with Amazon, Shopify, eBay, Etsy and TikTok Shop sellers day to day will already know where stock valuation tends to break down and can build a process that catches it before your year end rather than after.

At NS Accounting, this typically means agreeing a consistent stock valuation method with you at the outset, setting up bookkeeping processes that capture landed costs properly as stock arrives rather than reconstructing them months later, and reconciling marketplace inventory reports against your accounts at each year end so your cost of goods sold for ecommerce sellers figure is defensible if HMRC ever asks. For sellers scaling quickly and needing more than compliance, our CFO services extend this into stock forecasting and margin planning across product lines. If you are not yet sure what to look for in a specialist, our guide on how to choose an ecommerce accountant sets out the right questions to ask.

Getting Cost of Goods Sold Right for Your Ecommerce Business

Cost of goods sold for ecommerce sellers is not simply a compliance exercise to satisfy HMRC once a year. Done properly, it is the number that tells you which products are genuinely worth your time, whether your pricing actually covers what stock really costs once landed costs and prep fees are included, and how much cash you truly need to fund the next stock order. Done poorly, it quietly overstates or understates profit, distorts your tax position, and leaves you making decisions on numbers that were never accurate in the first place.

Whether you sell on Amazon, Shopify, eBay, Etsy, TikTok Shop or all five at once, an accurate, consistently applied approach to cost of goods sold for ecommerce sellers and stock valuation is one of the highest value habits an ecommerce business can build. If you would like help setting up a proper stock valuation process, reviewing how your current cost of goods sold for ecommerce sellers figure has been calculated, or preparing accurate year end accounts, our team at NS Accounting works with ecommerce sellers across the UK every day and would be glad to help. Get in touch to talk through your stock and cost of goods sold for ecommerce sellers setup.