The first reason is the money issue. Such a sale will happen only after bargaining. The buyer, who is your friend, will be hoping for a good discount. But this should not lead to the fact that you may lose part of the money when selling. Such situation will simply lead to the disorder of your relationship with the buyer, which is your family.
The second reason could be a technical malfunction of the car. The machine can break any second, and with the perfect auto may after a while need to be repaired. The reason for this may be rough handling, or just a coincidence, but everything will be perceived as deception. This will significantly spoil the relationship with the person who bought the car.
And the third reason is the request of the buyer about replacement of any component, be it a mirror, glass or the wipers. Explaining that when buying the car was quickly and poorly inspected. And you, in order not to upset family member and not spoil relations with him, will still have to change this detail. Like not the owner, and the time and the money will flow into the vehicle the new owner.
Thus, it is not necessary to sell a car to a relative, colleague or friends. This is the worst option of all, but it's good that he's not the only one.