I have, and probally will continue to.
I've kind of lived by the philosophy that I can always find another copy down the road, and and it will probally be cheaper.
I've only regretted selling two games. The first one was Pokemon Platnium because now I don't have a 4th gen game at all, and it's putting a whole in my pokemon collection because of it. At the time though, it was un-fun and I wanted to move on to another game (Dragonquest IX)
The second was Sonic Generations, but that's only because I'm a giant Sonic fan, but to be realistic, I didn't have a system to play it on, and it was the game that got me the most money that day.
Alot of the games I've sold in the past have slowly returned to me one way or another. Some recent examples include Metal Gear Solid 2, XenoSaga, and The Tower of Druaga: Fushingo Dungeon, which I payed no more then 6 bucks for all 3. Which at the time I probably sold them for 15 bucks each.