Currently Gamestop makes the vast majority of its profits off of used game sales. Gamestop typically pays a small amount for a used games it buys from a customer - much less than it pays game publishers for new copies. It then sells the used copy for nearly the same price as a new one.
This works great for Gamestop, but probably not so great for publishers. The above transaction gives money to the used game seller and Gamestop, but not the people who created the game.