> Pawn shop for nerds/the middle class.

That's kind of but not really what GameStop was before digital games took over and isn't what eBay is. A majority of Pawn Shop business is in the name pawns and neither eBay nor GameStop have ever done.

> They could build AI to help authentication and just identifying sellers would help tremendously with fraud.

That's a huge step beyond reasonable, there have been billions spent on the current AIs and none of them are up to the task of telling a good fake from a real item. Maybe one day but that's ages away.

And fraud doesn't just happen on the seller side, a big headache for sellers is buyer fraud where they'll claim the item is wrong/broken and there's almost no recourse for sellers to prove that's incorrect. eBay could just as easily partner with UPS stores to implement seller and buyer authentication anyways there's not a huge benefit to GameStop in particular being the partner there. It's always been an option and they haven't taken it.