I've never sold any of my games - they're all at my parents house, and will be in my house when i get a bigger place. I guess i'm a bit of a hoarder, coupled with the fact that i don't tend to have many games anyway.
I have given a few away though. Gave a couple to my cousin to play (he's about 7 years younger), and gave a load of shovel-ware to charity shops.
It always seems like too much effort to take your games and sell them for like £2.50 store credit, which, like Happenstance said, you might regret in the future.