I bought a used copy of SC2 and have been unable to activate it; I only write this so that resellers and buyers of used copies of SC2 are forewarned. Blizzard are slow to respond to emails and phone calls, and when you do get through to them be prepared to be fobbed off with all sorts of reasons as to why it is virtually impossible to transfer the license between battle.net accounts.
The long and the short of it is that Blizzard want to make more money and reduce piracy to such an extent that they have put in place such a burdensome process that most will just give up and buy a full price copy. As a seller you have no option of deactivating your license key, and as a buyer you cannot influence the transfer of an activated license to your battle.net account. Blizzard obviously have a process in place to handle such issues, but they have made it hard to discover (can you see it on their website?) and they support line will not explain it for fear of weakening the process. As a reseller you will have no knowledge of the buyers battle.net account details (why should you) and it was explained to me by a Blizzard support supervisor that for data protection reasons the reseller should not have access to such information, and yet I was told that because cases of license transfer are so irregular that the only way this can be achieved is through both parties contacting Blizzard and providing the details which they shouldn't have!
All I want to do is have a legitimate copy of SC2 to play on my laptop for when I'm away from home. If you want the easy option then don't try to save money buying a used copy of SC2; save yourself the hassle and pay full price.