Remember that CAG.com is a somewhat isolated community. We are mostly legitimate buyers unwilling to pay full price. This means we're as a whole not supportive of piracy, nor $60 per game. That said, I support used games sales, and here's why (many of them already mentioned):
1) Used games don't just materialize. They were once new games.
2) Used games are purchased by people that wouldn't buy them new, even if used weren't available, usually in the case of buyer uncertainty regarding the game's quality.
3) Remember that the video game industry is, in fact, and industry, and that you can't completely separate its components. When GameStop does well, GameStop buys more games, which in turn means profit for distributor, publisher, and developer.
4) Because of this, any activity related to games (used at GS, used on eBay, new, etc) ultimately means more profit for the ENTIRE industry indirectly. Used activity is better than no activity.
5) As mentioned, consumers venturing on used games turns into brand, dev, and publisher recognition (as well as store loyalty). For instance, had I not bought Devil May Cry 1 and 3 used, I may never have bought DMC4 at all. And I would not have bought DMC1/3 new (partly because it would not have even been available). Anyone in marketing can tell you that brand recognition of any sort is an EXTREMLY important factor in sales. It cannot be underestimated. Imagine what fraction of sales BioWare would've seen on Knights of the Old Republic if it weren't a Star Wars game.
6) Also as mentioned, it is not the consumer's responsibility to act in the dev/pub's interest. A publisher shall request nothing from the consumer and thank him or her when they do support them. This is basic marketing and basic customer service. If a business is begging you to help them by acting outside your own financial and entertainment interests, they are not good businessmen and are probably throwing away what profits you did provide them with by using other poor business practices.
6b) With the above said, it is reasonable to direct your finances such that they profit your business of choice. Given similar price tags, it makes sense to choose the purchase option that profits the developer (e.g. don't buy a used game at $55 instead of a new one at $60).
6c) (rant) I'm sick of bleeding heart gamers talking about "support support support!" If a developer can't provide you with entertainment for a reasonable price now, why on EARTH would you throw more money at them so that they'll gouge you again later? (Certain anime DVD prices come to mind). Instead, promote the general purchase of your favorite game, not buying it new or from a certain place. More sales is still more sales. It's not like the publishers don't know how well a game does post-production.
7) Reducing or eliminating used game sales would not significantly decrease the cost of a new game. Prior to used game sales popularity, rentals were much more popular. Game rentals are still pretty popular today, and devs/pubs/dists support rentals because it's profitable.