So.... he's had 'em since he was a kid and he has a LOSS ???  

You imply he's an adult now.

I doubt, very strongly doubt, that there is loss.

AND, bonus question, what would the tax rate be?

Well, he hasn't had them *all* since he was a kid. He's been an avid collector in recent years as well. It's just a thing he's accumulated over time. I don't doubt that overall he showed a loss - it was a big collection, and considering the size it didn't sell for much by my reckoning (I'm no expert in baseball cards, though). All I really have to work with at the moment is the 1099K and his statement as to what he spent on it (he has a spreadsheet showing it). He's solidly in the 22% bracket for 2024, filing single.

The ones he bought as a kid may have gone up in value and should be reported on Schedule D as a capital gain.  The ones he bought more recently may have resulted in a loss, which is not deductible because they are personal items, not investments.  (If they had been investments, he would have been keeping better records.) 

EDITED:  I posted this before seeing your reference to a spreadsheet.  So which is it?  He doesn't know how much of a loss he has, or he has exact records?  And the problem is that with a bulk sale, he didn't break down the proceeds per item.  

There's a place at the top of Schedule 1  "For 2024, enter the amount reported to you on Form(s) 1099-K that was included in error or for personal items sold at a loss."  Not sure how to get there with the software, I haven't had to use it yet.  

He's solidly in the 22% bracket for 2024.

Suggest you research 'Collectibles' and the taxation of them.

At least for the ones that do show a gain.

That's the main issue - he has a good grasp on what most of them cost, but he sold them as a group, not individually.

I would add up the total cost of those he knows, using zero for those he doesn't, and divide that into the proceeds.  Then prorate.  For example, $10,000 cost and $8,000 proceeds, the basis for everything is 80% of cost.  Then decide if you want to call the losers "investments" or "childish pleasures."  

@abctax55  The taxpayer has likely had huge dental expenses from chewing all of that bubble gum in his youth. His mouth was full of silver amalgams that have fallen out or led to cracked molars which have since been replaced with dental inlays or caps. The taxpayer definitely "lost" financially on his card hobby.