Retailers connect customers Used Games with customers. Consumers get money or credit for their games. GameStop is an intermediary that takes on the costs of purchasing games from walk-in customers in an attempt on aggregate to make money. Some games may lose them money, some may have huge margins, but they provide that service.
As someone who despises a lot of GameStop's techniques, I can't throw everyone out because GameStop charges too much, but I don't think that is Microsoft or Sony's business to control.
This is not only about protecting customer's rights to sell their games to GameStop, but also to maintain the utility of the physical products we purchase. So that we don't set a precedent that allows companies to continue to control how we use the content we physically own. I should be able to have the choice to lend my game to a friend or sell my games without having to ask Sony or Microsoft permission to. I shouldn't have to be at the mercy of their economic stranglehold on the used games market.