Sorry, no. Civ V is Steam-only (only exception is the Mac App Store version) and if the activation code has already been claimed by another Steam user (which should be no surprise, I guess) you can't claim that code and therefore can't play that instance of the game.
On the other hand, if you could have claimed that code, it probably would only have given you access to the base game (often referred to a "vanilla") and not the expansions (Gods & Kings and Brave New World) or any of the various DLCs. You should be able to find the complete package on sale -- sometimes on Steam sales and often through other electronic retailers (it will probably be more than you paid at the flea market, but still deeply discounted).
