There is no reason why a consumer cannot lend money to another consumer and charge interest.
However, it would be better for clarity increasing the sales price of the car.
I think it would be a very risky thing to do. It's very difficult to enforce consumer debt and very dangerous to repossess the car.
You could ask for 6 post dated cheques. Then you would not have to prove in court that the buyer owed you money. You would get a judgment for the amount outstanding almost automatically. But trying to collect it would be very difficult.
Brendan