You mix some things up...Diablo COULD be easy to sell to someone else if they didn't force you to connect it to you bnet account...Another GOOD way to have a resellable Diablo3 would be to start another Bnet account and ONLY put your D3 there...then you resell the account...
For mmo's it's trickier and not really possible...Because the former owner can always call Blizzard/Bioware/Funcom and such to claim back their account falsely saying they were hacked...
So I agree that offline single player games and even games like COD/BF could be sold again...But for mmo's forget it