I've always been a fan of buying used video games and used video game systems. Since most games are only published and printed for a limited amount of time and each video game hardware system is only made in a limited amount for a limited period of time getting buying used stuff is often the only way to find anything older than a few years. However there is no way that I would ever buy a used Xbox 360.
Customers simply have no way of knowing what that system has been through and the state of the internal hardware. Since the Xbox 360 has design issues, parts supply, manufacturing and hardware repair problems there is no way that you can rely on getting a reliable system. Unless you know how an Xbox 360 system has been treated, used of if it's had any previous hardware problems. There is no way of knowing if it's been repaired once or a dozen times. Buying it from GameStop,Game Crazy, Ebay, Craig's List it is all the same. Places like GameStop will only test to see if an Xbox will boot up once or twice while they are deciding to accept it but they have no idea how reliable it is going to be in the long term. Getting a "repaired" system back from Microsoft is almost just as bad from getting one from a used lot. I would never ever buy a used Xbox 360 system and I think that places that sell used Xbox 360s are not being responsible to their customers.