Besides duplicates and direct ports, the only games I ever remember trading in are a handful of outdated Pac-Man/Namco Museum games, and Donkey Kong Land. No regrets there. Shoot, I still have an old copy of G&WG3!
I personally prefer the oldies to most new games. (With obvious expections such as SSBM and SSBB) I'd be pretty dang mad if I ever lost my SMBDX cartridge, considering the countless hours that went into my roughly 2,000,000-point challenge mode score.