I think the difference is that GameStop already has this network of pawn shops, and those stores are doing enough business to justify their existence (at least somewhat; GameStop wasn't doing great before all the craziness). I could see something like the UPS kiosk in my local Walmart working; if you offered eBay returns at GameStop stores, you would be adding revenue without needing to justify an entire store to it like eBay would have needed to.
There's also the synergy that GameStop now has access to more used gaming inventory, a category that I'm led to believe is high margin for the stores.