Gamestop knows they serve no purpose in the gaming world anymore. What's the point of buying physical, when it's just a disc with the game version 1.0, and manditory day 1 updates? It makes it essentially a digital download with ewaste.
Ebay on the other hand has a future still existing.
Right now gamestop is a glorified novelty shop with a video games and pop culture theme. They know that business plan won't last.
Ebay on the other hand, I can't see any reason why they would be interested in anyway in this lowball offer.