I bought a pre-owned battlefield 3 and the person I bought it from already used the code (game was from gamestop) what should I do?

OP check your Gamestop receipt, they are giving out free online pass codes with used games after losting a California court case.  I'm in Virginia so they're not just providing codes in California.  I purchased a used copy of Uncharted 3 and they provided the code free of charge 3 days ago, if the code is not on your receipt take it back to Gamestop and ask for it. 

Failing that you may have to purcahse a new online pass for $9.99 since technically EA isn't mandated to provide a second code for free to a disc.