True, but EB/Gamestop have huge incentives to bring new games back for trade value. When The Godfather was released, there actually was a promotion that if you could beat the game within a certain period of time (I think it was either a few days or a week), you'd trade it back in and it was 'free'. Nowadays, they have their "Most Wanted" lists, with new releases fetching the biggest bounties, and new releases tend to have a "trade 3 and it's free" promotion as well. In other words, EB/Gamestop have put HUGE amounts of effort into making people part with new releases in order to get other games, in a rather turnstile fashion. If you've ever been into an EB or Gamestop, you'll probably notice that there actually are a surprising number of new releases with used stickers on.
It's true that on day 0, there shouldn't be many used copies, but I have witnessed it, in particular with Halo 3. Within less than a week's time, used copies lined the shelves for $5 less than new.
I don't believe that it's fair for a product to have to compete with itself on the same shelves that it's on. Recently used copies are very likely to not have any damage or defects, and EB/Gamestop warrant them as working for a certain period of time, anyway, so there isn't any incentive to pay the extra to buy a full retail copy. This is part of the reason why multiplayer is becoming such a huge cornerstone of game development - Multiplayer communities generally mean that people who play online will continue to do so, rather than trading the game in. It's also why EA and others have been bundling one-use DLC with their games, and why it could very well be possible that in order to combat both piracy and used game sales, one-use (or limited-use) serial keys for console games might be in our future. With that in mind, it would be a LOT better to come to an agreement with publishers over street dates rather than having the bottom taken out from under the entire used game industry in the name of anti-piracy (which is exactly how it would be spun).