To me what has ruined GameStop is stop treating customers like they are idiots. The rare times I go in to one I am always told that 3DS and PS3 games will only work on official hardware.
There is a market for used game, especially retro games. The Exchange here in Chicago most of their business is PS2 and 3 games, as well as older retro games and consoles. According to one fansite on average 750,000 play their PS3 online daily. But yet GameStop constantly refuses to accept this fact. There are also 4 retro game stores in the Chicago area.
One time when they were still selling PSP games, even though 1 title had the store’s label it was listed as new.