I don't see how this exchange of money makes used games sales better than piracy.
I remember that back in the day, long before broadband, there were stores that sold pirated copies of games for a fraction of the price. We were paying them money, they were giving us the game. They had already paid for the game, mind you; they needed a legit copy to start copying it.
Or what if you're "donating" to your favorite torrent site or group of crackers? They bought a legit copy, they gave it to you, you paid them.
The only difference is scale, as a previous poster said. But for the law, scale changes the penalty, it does not make something legal. Stealing 10 dollars and stealing a million is still a crime. You get reprimanded for the first, you go to jail for the second (or get elected in office, but that's another story).
Another thought: We really need to start getting away from the physical copy mentality. Things are going away from that and in a few years or more, services like Steam would be the only ones available in any platform. Owning software is not the same as owning a table...not at all. A table cannot be copied at no cost and two tables cost twice as much to be made. A game developer can "burn" 3 or a million DVDs and the cost would be about the same (since packaging is negligible).
Another thought: You do realize it is illegal to lend your copy to a friend of yours, right? How many avoid doing that for this reason, though?
What is illegal is not necessarily immoral and what is immoral is not necessarily illegal. Food for thought.
PS: I don't support piracy, in any case, but I would never stop you from doing it, though.