I've sold a few games. The only one I wish I had again is SpongeBob Squarepants: Battle for Bikini Bottom for the Game Boy Advance, which I got as a present when I was ten. I'm not against selling games on principle, but unless I am pretty much totally sure that I will never want to play a game again and that it will be reasonably easy to obtain another copy if I somehow do, I won't sell it. I still own Star Wars: Rogue Leader, which seems to have gotten great reviews but that I've only played a few times despite owning it for almost eight years.
I've never sold a Pokemon game, for that matter, though I've lost a couple of them.