The 1099-K will report the gross payments paid by the buyer on your behalf. You must deduct any refunds, discounts, fees, shipping, and other costs of the sale.
Not included in the 1099-K are: Sales tax, international shipping, or customs that eBay collected directly from the buyer on eBay's behalf and that were never assigned to the seller.
eBay uses the settlement date (when the buyer payment clears and it moves to your side of the accounting column) to determine the tax year for your transaction. That means if you have a sale on Dec 30 and the buyer pays on Jan 2, the transaction is reported in the next year. Similarly, you may have a settlement date this year, and would need to find the sale in the prior year. So when reconciling your 1099, be sure to look closely at transactions at both ends of the calendar.
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