I sell everything (Except downloaded games of course) once I'm completely done with it, otherwise it ends up gathering dust on my shelves and I mostly never play it again if I kept it. At least by selling it, I get some money back to buy other games, it's the circle of life... for video games.
Only case I regretted selling a game was when later on that game got DLC's that I would have cared to get but since I sold the game I couldn't play the DLC's anymore. As far as I can remember this only happened once to me and that was with the first Evil Within game. Would have loved to do the DLC's but never did them cause I sold the main game.
It probably will never happen again as I never buy games day one anymore. By the time I get my games now, every possible DLC's I could care for have been out so when I do sell those games there are no regrets.