PC games do not work that way. PC games have not been viable to buy used since.....2004? Earlier, really, with cdkey reuse and whatnot. You cannot buy a used copy of Starcraft 2 and play it. The cdkey is required to activate (play) the game and the cdkey gets bound to the Battle.net account it was first used with. It cannot be unbound.
No, you cannot (through legitimate means) play Starcraft 2 offline perpetually. You will, from time to time, need to go online to extend the length of time available offline. I think it lets you play offline for two weeks or so.
There is no LAN mode offline (for Multiplayer) as well as most other non-campaign modes, so offline mode is of limited use.