I am not a lawyer but I have been reading up on this lately and here is the way I understand it as regards US law. One of the reasons that the First Sales Doctrine applies to physical media is because it inherently degrades over time (a used console game disk can become scratched for example). This is not the case with Digital Distribution. A used purely digital copy of a game is in identical condition to a new copy and therefore it is exempt from the First Sales Doctrine.
Furthermore, purely digital media files like mp3s, movies, computer programs, etc... can be copied and/or backed up perfectly an unlimited number of times very easily. This is one of the main arguments I have seen as to why downloaded media is exempt from the First Sales Doctrine, it would be possible for people to buy the game, song, or whatever and then distribute it infinitely by just making copies and reselling as there really is no original copy. This infringes on the copyright holders reproduction rights.
Finally:
http://en.wikipedia.org/wiki/First-sale_doctrine
So basically, it seems to me that current US law mandates that consumers must be allowed to sell used physical video game console media. A complete ban on selling used games on physical media would be either outright illegal or fall into a gray area that the courts will have to deliberate on. But again, I am certainly not a lawyer.