For modern games, I keep the ones I enjoy for as long as I enjoy them. They are here to entertain me. And once I get bored with a game, it goes on the block. And once it does, they usually leave quick.
Classic games are different. I own a few that I LOVE playing. But because I also host a Classics league, reliability is critical for me. A game needs to play all night long, and if it fails during league a couple times, it gets tossed.
I don't care about the opportunity cost of owning them. In my mind, they become worthless as soon as they get here. If I could double the money I have in a game, who cares. It won't impact me that much, and I won't have the game anymore. Stars is my classic example of that. I could make money on it if I sold it today... but then I wouldn't have Stars. The cash in my pocket will never get me the same fun I get from Stars.