I'm actually a pretty good mix. I don't mind used, even incomplete. If I'm saving enough (usually part of those buy 1 get one half off, if a game doesn't have an instruction manual, it doesn't really bug me (except in rare instances). But, if I'm only going to save $3, then I'll just buy new, and not have the issue.
The problem at Gamestop and EB is that, since they open the seal, I rarely ever buy new games there. Even if they are truely new, they really don't feel new.