is rego and VIN not required to advertise on carsales? maybe he wants to verify the details.
also, he may want to check to make sure its not an encumbered vehicle, more likely the bank wants to.
as already mentioned, if he pays by bank cheque, call the bank to confirm its legit. maybe speak to him before he goes to see the car and let him know the cheque will need to clear before he can take delivery of the car. the same thing would happen at a dealer.
theres nothing wrong with paying cash, been there done that too! $21,000 was counted, i drove away. fastest transaction ever.