The single most expensive misunderstanding in ecommerce taxes: a big purchase order is not a big deduction. When you wire $30,000 to your supplier in November, your bank balance drops but your tax bill does not. You traded cash for an asset. The deduction comes later, unit by unit, as that stock actually sells.
Cash out in November, deduction next year
Tax accounting treats inventory differently from ordinary expenses. Ad spend, software, and shipping labels are deductible when you pay them. Inventory cost is capitalized: it sits as an asset until the matching sale happens. So a seller who loads up on stock in Q4 for January demand gets no deduction that year for the unsold units. The units that sold in December are deductible in December's year; the rest carry into the new year at cost.
This is why an ecommerce profit-and-loss can show a healthy profit while the bank account feels empty. The cash went into shelves of product, but for tax purposes you only "spent" the portion that customers bought.
How COGS turns stock into a write-off
The mechanism is cost of goods sold. On a sole proprietor's Schedule C, Part III walks through it: beginning inventory, plus purchases during the year, plus freight-in and other costs to get product ready to sell, minus ending inventory. The result is COGS, and it subtracts directly from your gross receipts before profit is calculated.
That formula is why the year-end inventory count matters so much. Ending inventory is what you still hold at cost on December 31. Overstate it and you understate your deduction; understate it and you overstate the deduction and invite trouble. Sellers on Shopify or Amazon should reconcile their software's inventory valuation to a physical or FBA count at least once a year. Unsellable stock is not stuck forever: damaged, returned-unsellable, or disposed goods come out of ending inventory, which increases COGS in the year you write them off or destroy them.
Cash method sellers and the gross receipts test
Ecommerce sellers often ask whether being on the cash method changes any of this. Mostly, no. Small sellers who fall under the IRS gross receipts test can use the cash method and account for inventory as non-incidental materials and supplies, or follow how their own books treat it. That sounds like relief, but non-incidental materials and supplies are still deducted when used or consumed, which for a retailer means when the product is sold, not when it is purchased. The paperwork gets simpler; the timing rule survives.
Larger sellers past the gross receipts test use the accrual method with full inventory accounting. Either way, the planning takeaway is identical: buying stock in late December does not lower this year's taxes. If you want a year-end deduction, prepay deductible expenses like advertising or software, or clear out dead stock so its cost finally lands in COGS.