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.

That's a good point that no one brings up.  It was already bought, so it's more like this object that was already used is now getting used again.  If anything, you have to look at it like: would that person have bought the game at full price if there were no other options?  Possibly not.  I know I wouldn't have a good number of the games that I currently have in my collection without them having been on sale during close-out sales or used game sales.  Gaming is a bit expensive for me so that's sort of how I roll.