Some retro games have some value to it so I would strongly recommend to do some Research on them, as for Modern games I would look (forgive me for saying this) in Gamestop website for Trade Deals that you can the High values in Store credit for future use. If you want.
P.S: I believe Gamestop won't accept GameCube games anymore.
"There is no Real ending. It's just the place where you stop the story."