This boils down to one sadly forgotten thing called "end user rights".
As consumers it is our right to sell/trade/rent the tangible physical content of products we purchased. At any price we like, to anyone we like. Regardless whether DRM prevents second-hand owners from using them for their perceived intended purpose.
If I want to sell used boxed World of Warcraft copies or Xbox Live subscription cards and am able to find buyers who don't care they're 'useless'... I'm allowed to do so.
Besides Gamestop is only current leader due to market forces. They weren't the first popular game chain. They won't be the last. Nobody's stopping a different company from opening another comparable brick and mortar chain who could take away their business. As steep as the entry barrier is. You'd need an ad campaign large/convincing enough to sway people over. Along with comparable enough service/prices/selection to maintain interest, and store locations in similarly enough convenient places across the country to win nationally. Etc. Not easy!