It appears the United States Court of Appeals for the Ninth Circuit agrees with Penny Arcade: You really can't sell used software legally. More specifically the court said yesterday that the first-sale doctrine, which e.g. allows you to sell used books, doesn't apply to "those who are only licensed to use their copies of copyrighted works". The judges upheld the legality of shrink-wrap and click-wrap software licenses, so if it says you can't transfer or resell that video game in the legal text you clicked through without reading while installing the game, that is legally binding.
Basically the court's opinion is that just because you are holding a physical copy of a copyrighted material in your hands, you aren't the legal "owner" of it. You only paid for a license to use that copyrighted material, and the company which licensed that software to you has the right to limit what you can do with that copy.