Try selling it on ebay or a normal person. Gamestop will rip you off. I was in same position as you back in august, but I didnt sell my Wii because I think you have to pay to sell things on ebay? So I just kept the Wii and bought a 360. But I had money so o.O
You could just keep it, unless you dont have money and are in need of it. If so, you will get the best offers on ebay because people are desperate.