Word of advice: Don't sell gaming consoles/games to Gamestop. They tend to rip-off their customers, as it has happened to me and many others. If you have no other choices like Ebay or Craigslist then it would be an absolute last resort.
Well that's just my opinion, anyway I think you should go with Ebay with no reserves and free shipping, I've seen prices for those auctions climb higher and higher by the hour. (from a year ago, when there were a huge shortage of Wii's) And you can probably get around $220 or so when you sell it, it depends on what you include with the original accessories, I bought mine for $180.