If the 'seller' didn't own the car, he isn't authorized to sell it. If he completes the transfer of title to his name, then he can subsequently transfer the title to you.
If the 'seller' wants to go with you to a DMV office to complete successive transfers (his seller to him, then him to you), that may be possible -- but you'll have to check with the DMV to see if they can instantly process the first transfer.
If you cannot get title, you have the option of suing for the purchase price (and returning the car).

