Well, for one, selling stuff at a yard sale isn’t non-taxable. But generally you don’t have to pay tax because you’re selling things for less than you bought them for (you have a personal loss that is non deductible). If you sell it for more than you paid, you have gain that is taxable (but often unreported). But in any event, 1099-K reporting is only from a payment processor - if you accept cash where there is no payment processor involved there wouldn’t be any reporting. But if you sell some stuff on eBay, you might get a 1099 from them or PayPal. Then, you will have to show the proceeds from the sales and your basis, or the IRS matching program may indicate that you have unreported income.