Obviously, Gamestop needs to take a slight profit to stay in business and be motivated, but, while I don't know their prices (I've heard they are high.), they shouldn't be higher than an additional 5-10-15$, at most, I suppose. But they have no competitors -- so they get to rule the used games market. If they had a competitor, it would be better. 2 competitors, or more, could be good for the market.
A chain, like Gamestop, could go into business with a publisher/developer, or several, and let people trade 2 games for a brand new game of whatever publisher they are working with, and then proceed to sell the used games at a discount. Profits would be split between the chain and the publisher/developer. Could even work over the web in the countries the chain is located in, and they could let people mail them the games as well.