Once I got into my head that I was not going to be a set collector it has not been that difficult to sell off something I don't like when I try the game out for the first time. If a game doesn't click then it doesn't click, I'm not going to keep stuff that I don't like playing.
One big game that I ended up selling was metal gear solid, I was a total nintendo fanboy growing up so I didn't have a playstation and played it for the first time, a few missions and just couldn't get into it. I had no issue at all selling it to a friend that I knew would like that game and actually play it.