The WHT is on invoice price. If you are buying a car at own, the person on whose NIC it was booked and is being delivered has already paid the WHT against his NIC. And when you get the car registered you will have to pay it then. WHT is paid either when buying a new car on booking, or registration, or getting a registered vehicle transferred which is not older than 60 months or 5 years.
So probably the car you are purchasing at own will be on a second name when you register it, doesn't mean you will be paying his WHT and then yours again. Stay away from such thugs.
Some investors book a car, pay 75,000 WHT being a filer, then demand own and WHT for non filers i.e. 150,000 (taxes mentioned for 1.8) and being a filer they claim reimbursement of the 75,000 in their annual return from FBR. So technically they are getting a profit of own+150000 and the poor buyer pays it again when he gets it registered.
If the dealer is a 3S and says you can pay own and factory invoice will be in your name as booking owner, you wont be paying WHT again while registering in that case. That's a different story and an illegal one on his part. I doubt that happens though. Dealerships are only your communication to the head office. Bookings and invoices are direct from the head office.