if you have beat them then why not sell them? (exception for banjo kazooie)
A little something called "replaying". How can you sell all of your games once you beat them? If I sold all of the games I beat, I would've gotten rid of some of the great retro Sonic games years ago, and I'd be extraordinarily upset now; I love them. I would've sold Melee once Brawl came out, most likely, and then once Brawl sucked where would I be? GOD, I would've sold Metroid Fusion ages ago; I've played through that game at least 20 times now. But I really enjoy replaying it to try for better times, going for speed runs, 1% runs, etc.
In my opinion, you shouldn't sell any game unless it was so horrible you never intend to play it again (I did that for Pokemon Myster Dungeon: Blue Rescue Team. I never beat it, but it was just...ugh, I didn't like it.) So in my opinion you should keep them all.