It's a 2nd hand car, in addition, it's a converted car... I agree that the fact that you guys were given more than ample chances to test the car and the buyer was satisfied after the "checks" should mean that it passed the buyer's "checklist" already.

It makes no difference between a private sale, 2nd hand lot sale, etc... It's a used car, it may be fine during testing, but there's no way the owner can predict what'll happen to it a day, a week, a month, down the road...

The buyer bought a car that's known to be used by kids or racers (best case scenario), it's converted as well... The risks are laid out pretty clearly, shouldn't have purchased the car in the first place.