The link makes the policy clear, but not how items sold at a loss get documented.
I buy a couch for $1200. Several years later I sell it for $800. No tax required. But I don’t keep old receipts so can’t actually prove I paid $1200. So now do I need to pay tax on $800 income?
I buy a part for $50 cash at a swap meet. I find out after I get home it’s the wrong part, I already had one and forgot I did, or later I find a better part to use instead, so I sell the one I bought at the swap meet online. I only get $40. I didn’t get a receipt when I bought the part, so do I owe tax on $40 on top of losing $10 on the part?
A record keeping pita that makes it more trouble than it’s worth to sell online anymore.