Bro i think for being on the safe side make sure u have the following documents when u make a car deal:
1) Original Sales Invoice (Jo company ke invoice ho ge)
2) Sales Receipt
(Which mentions how much u bought this car for. This is another proof that u bought this car from Mr.X on this date for ..... Amount)
3) Original Registration Book
4) NIC copy of the owner
5) Transfer Letter ( For change in ownership)
6) Delivery Letter
( To have a written proof that the car is in ur custody. It takes few days before the ownership transfers so this letter is made to stay on the safe side)
7) Tax documents( Make sure there are no dues)
Verify the ownership of the car by calling CPLC. Verify the person's ID card from Nadra. Get all the necessary things verified before finalizing the deal.
I hope u have a better idea about the documents required. Let me know if my msg helped u out.