Cant activate iPhone 5?

Does anyone knows what is going on?

I am current AT&T customer, I did purchase AT&T iPhone 5 from craiglist, phone is brand new and it was not even open. Now I did try to activate phone and its asking me for last 4 ssn and zipcode from the person it was purchased from? Now i dont know his information, and I did call the store and I was told they cant help me out unless that person first activates phone.

Is this true? I cant get new SIM card for iPhone 5 and activate it under my name?