The problem is to everyone else, their incessant pitch to every customer at least twice per interaction hurts the developers. As if someone buys a game new for 60, then sells it back for 15 credit in 1 week, the 2nd person buys it at 45 and Gamestop pockets $50 on that second transaction. This means they get to double-dip and the developers sell fewer copies.
The game companies got around it a bit with inbox DLC that could only be used by the original buyer, and the 2nd buyer had to pay 15 bucks for ensuring the developer made $15 on the second purchase.
I used to hate my yearly trek to Gamestop HQ and have to pitch my titles and promise them they would get the best DLC. TBH most people I knew that were in that position almost always gave Gamestop the worst preorder DLC exclusive on purpose and lie that it was the best lol.