The car is taken possession of by acquiring it and moving it from where it is, even if the official registration has not been completed
Question

                If I buy a car for cash and pay the price, and that is recorded in the showroom’s records, is that sufficient for taking possession of it, or is it essential to transfer ownership officially? Please note that I intend to sell it.
Praise be to Allah.
    If the sale has been completed and you have taken possession 
    of the car, then you may dispose of it, but you have to take it out of the 
    place where it was sold to you, to your place, or to some other place, or to 
    another showroom, so that you will have taken possession of it, because the 
    Messenger  (peace and blessings of Allaah be upon him) forbade selling 
    goods where they were bought, until the merchants took them to their own place. That applies even if the rest of the procedures have not yet been completed, so long as the transaction between you two has been completed. Once you part, then the sale becomes binding. End quote.