As far as I see it, if someone is buying a game used, that means it was once bought full price. So the publisher isn't losing money there, and the new owner might become interested in that game and buy the follow up when it's realeased, new. If the game wasn't cheap due to use, then that buyer wouldn't have bought it... because they didn't when it was new. Phew!

Anyhow, if the game is selling well used, it must have sold well new.