As reflector guy (and possibly others) pointed out, if you use eBay and other methods where you sell via a wire service (Paypal and such) you will be 1099 for all transactions. It makes no note of whether you bought an item then sold it for less, it will see that sale as a profit in your pocket. IDK if there is some method of accounting that can be made to balance the sheet (as it were), but I have discontinued ALL methods of selling items that require me to be wired money to make a sale. My side hustle selling old office PC for instance just wasn't worth paying to do. I swapped over to CL and cash only deals, which although I make WAY less doing so, I also don't just have to turn around and give that money to the tax man.

YMMV