Sales of used games keep game retailers in business. All of them say it. Used games give them most of their profits but are also much smaller sales compared to to new games. I think it's like 10% used sales, 90% new.

Also for the fact there are those deals like "Trade 2 games in and get RE5 for free" kind of things or using trade in credit to buy a new game.

It's not a problem and in fact helps game specific retailers a lot.