I say dont sell it to the kid not for safety reasons, but simply because you may always have to hear things after the sale. Being that the car is modified to a level that most normal service shops will refuse to work on, he or the mother is going to call everytime something happens to the car, and that sucks, aks me about that one. Being that the possible buyer is under 18 probably means the mother would be the one calling you for help, and that just sucks. True, there is a safety issue involved as well and it can contribute to not wanting to make the sale, but there is also the factor of what also could happen after the sale selling the car to a minor who doesnt know the extent and extremity of modifications done to said vehicle.