Do not buy StarCraft II used. You will not be able to play.
StarCraft II is only playable via a Battle.net account with rights to play StarCraft II. There are 2 ways to get those rights:
- Purchase the game online via the Battle.net website.
- Purchase a boxed copy of the game and register the 1-time use key with your account on the Battle.net website
If you do purchase the boxed copy used, the 1-time use key will probably already have been used.
The only thing you could possibly hope to get from purchasing the game used is any of the physical media that comes in the box (the manual, the disc itself). If you haven't purchased the game already, chances are you don't care one whit about any of that stuff anyway.