You were scammed. Sony doesn't permit PSN accounts to be sold, traded, or given away, anyway. Even if he gave you legitimate information, he could have taken the account back at any time by calling Playstation customer support, and telling them the date-of-birth and secret question answer registered with the account. You can't find those things out by signing-in to PSN, and they can never be changed or updated either. Sony might even have banned your console if he accused you of hacking his account. Let this be a lesson to you. Stop cheating and earn your ranks. You got off easy losing only $20.