GameStop wouldn't take it back because they can't take your word for it, where they themselves know that it's effectively "new" even if it's open box for display reasons. Consumers can somewhat reasonably assume that they aren't selling you a used product as new due to potential false advertising claims.
An item that is open but never used is still new. A new car that has yet to be sold to anyone but that has been on numerous test drives is still new.
Yeah big difference between some random seller and an actual company that can get the book thrown at them for being shady. New requires closed box for p2p sales because you can't trust anyone at all. If I could somehow definitively prove that I've not actually played a game and the only thing removed was the plastic covering then, yeah, I could easily sell it as new.