The question is, what is income?
If you are only selling personal items for less than you paid, none of your sales proceeds are taxable income. If you sell items for more than you paid, the difference (gain) is taxable.
If you report your sales as a hobby, you would report the sales as capital gains or losses on schedule D. The losses are not deductible against other income but they are not taxable; the gains are taxable.
If you report the sales as a business on Schedule C, and you are only selling personal items for less than they cost, this would be selling inventory for less than the cost, you have a business loss and you have no taxable income, even though you have gross proceeds. (However, selling personal items at a loss should not be reported as business activity because you don't have a profit motive. Reporting your activity as a schedule C business would allow you to deduct your losses from your other income like wages or a pension and that is improper.)
The issue is how to show the IRS that even though you have a 1099-K showing gross proceeds, that your taxable amount was zero. There is a way to do that on your tax return which you should ask about when the forms come out. In the mean time, you start by gathering your records and making a list of your sales that includes item descriptions, the date purchased, price paid, date sold, and selling price. Make your best guess if you don't have receipts.
