Actually there is no real used game market for pc games and it has been that way for a very long time, starcraft one was only allowed 1 copy to be on battlenet at a time the only difference now is that games are online so much of the time that they are much more limited.

The thing that I don't like is that you have to login to battlenet for anything single player (other than playing as a guest) and this is an issue I wouldn't accept for any other game than sc2.