there's a Wall Street Journal article about this.
Etsy, eBay or StubHub Sellers: Expect a 1099-K From the IRS This Tax Season - WSJ
If you don't subscribe, here's a relevant paragraph.
The tax rules for selling personal stuff online are different. If you get a 1099-K for something you sold for less than you originally paid, you wouldn’t owe tax, but you would need to disclose it to the IRS.
eBay doesn’t know if the stuff you’re selling is your personal property, if you’re selling at a loss or reselling items at a profit. The IRS gives examples of how to account for these sales in the instructions to Schedule 1. There is a new line item on the top of Schedule 1 where you can list any amount received in error or for personal items sold at a loss.