But the CD key is tied to the particular copy, so how would that happen unless the seller kept the game installed without the media (which AFAIK isn't legal)?
The point i'm making is that the original owner would want to keep the game, as opposed to selling it on so they could continue to play the online mode. If they were to sell it to someone else they would lose said online mode, hence it becomes an incentive to keep the game. The game isn't being stopped from being sold on due to DRM etc, it's just being kept cos it's good.
A good example would be a friend of mine that wanted to play Half-Life 1, i would have been happy to lend/sell him mine but as i was very much addicted to TFC (a Half Life mod) and didn't want to lost the ability to play it he instead bought himself a copy.