I... pretty much never returned a game in my whole entire life. I lost a few gems though, like Kirby's Dreamland and Super Mario Land.

Metroid Prime. Sold it because I got the trilogy on Wii. Bought the original again though. The controls are just better.

My SNES games.

Super Smash Bros. Melee. My younger self thought Brawl was alot better...

I've only sold a few of my games but I've never regretted selling any of them. I do however regret not selling Skyrim. I bought it when it was new, played it for a few hours and then never played again. Ever since then, I thought I'd give it another chance but I don't think I'll ever play it again.

Sonic Colors

My NES collection.

Cross`Edge...not that it was a good game(it was alright) but now it's a rare find and worth good money even used

EDIT: Stand corrected, it's pricey new but you can find it for a cheap price used

I didn't sell or trade it, but I did lend a friend my copy of suikoden 3 and never got it back.

I don't have any regrets. Once I finish a game, I'm done, no need for it anymore, I sell or trade them immediately.