It is important to re-evaluate the reasons you have for selling the car. You could sell the car at a fair price in order to help your friend out. Ensure that all the legal documentations of the car are in order. Make suitable arrangements for payment. You can opt for an installment plan or a complete payment.
Create your own form of a physical contract where you state the down payment and monthly fees, both of you sign it. Tell him to use a money order and to write what the payment is for and he can keep the receipt of each money order and you get the actual money order.