I think it all comes down to what sort of person you are.
Hardcore gamers, or collectors, would most likely opt out of selling old games. Or even if you just like to have memories or want to have an impressing shelf, but normally it would be the first option. A lot of the time, selling old games will get you a small amount of money, and that doesn't pay up for the joy it gave to you when you picked it up at the store for a mean price. It's nice to play an old game every now then, I pop onto my Color to play a bit of Pokemon Yellow here and there, it's the original Pokemon game and the first I ever played; this game is precious to me. People treasure games, and that isn't a bad thing, games are what makes us happy, entertain us and let us have a good ol' laugh with a bunch of our friends. I know someone who has every Pokemon game, boxed, on his main shelf and you see it whenever you step foot into his room, and I respect him for that.
But then you get the people who don't care about the old, in with the new. Sometimes people could just see it as pointless, they would rather have that bit of extra money to buy some of the new releases. I can't say that I haven't done this, because I have, I was a bit low on buying Guild Wars 2 so I had to look back at some of my older games such as Oblivion GOTY Edition and sell them, I think there are a fair few people around here who have done this. I sell my old games that I hardly ever played, simply because I don't really like them and would much rather have a bit of extra cash saving up for a game I do like.
Ultimately, it will always be your choice. For me, it will always vary. If, as I mentioned, it is a game that I don't like and need cash, I will sell it. But if it is a game I loved and prize, I would just keep it.