It depends on whether you are selling items on eBay as a business, hobby or simply selling a few personal items.
If you have an eBay business, read Tax Champ @Carl’s answer in First time filing 1099-K trying to figure out where to put cost of goods
Report the income from your 1099-K by selecting 'Personal items sold', once completed you can use the following steps to complete your tax return reporting.
To report the cost of your items
Keep in mind that a loss on the sale of personal items is not allowed. A gain on the sale of personal items is taxable and required to be included in your income.