When you sell something for $200, PayPal doesn't know if you bought it for $150 or $250 ... they don't know if you have a gain or loss. I think they're pushing that responsibility onto you, without deciding if you have a gain or not.
Years ago I sold things on eBay. Some items I sold for less than I paid to cash them out. Others were collectible items I sold for a profit. To keep track I made a spreadsheet similar to how stock sales are tracked: purchase date and amount, sell date and amount, profit/loss. I forget where I reported that hobby income in my tax forms, though.