Actually, Gamestop's website used to be great. They had a bunch of old stock, they'd update it the same time every morning with the new inventory, and I could spend an hour browsing for copies of older games cheap.
Truth is, for profitable older games, most people are going to sell direct to buyer rather than GameStop. And selling a cart-only copy of Cory in the House doesn't pay the bills. GameStop's entire business model is much like that of video rental stores and is destined to fail, eventually, period. Their best hope was to slowly convert to an all-things-geek shop, which they tried to do, but tried to do badly, stuffing tiny mall shops full of overpriced Funko Pops, giving zero shots about customer service, and, in my humble opinion, failing to embrace the expanding tabletop gaming market.
Where GameStop could have become an all-inclusive FLGS, they gave a very token effort towards having analog games while video games were devoured by the digital marketplace.