It depends. It sounds like you are in the business of selling items through StockX. Enter your 1099-K income on your self employment schedule, then you can enter the cost of the items you sold under 'Supplies' expense.
Under the Tax Cuts and Jobs Act (TCJA) Inventory is no longer required to use the accrual method in the following situation:
The TCJA allows small business taxpayers with average annual gross receipts of $25 million or less in the prior three-year period to use the cash method of accounting. The law expands the number of small business taxpayers eligible to use the cash method of accounting and exempts these small businesses from certain accounting rules for inventories, cost capitalization and long-term contracts. As a result, more small business taxpayers can change to cash method accounting starting after Dec. 31, 2017.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
