I think the problem is that you have been to good natured. Once he started messing you about you should have told him to sod off.

IMO you make the effort but once they let you down, regardless of reasons, THEY have to make the effort to come to you. Then when they have made the purchase - if your not a trader - it is sold as seen, their responsibility to check the car over.

Once you help them once, they will be back time after time.

Tell him you have done more than you ever should, that he has been killing the thing and that you are not surprised things are breaking if he is not treating the vehicle with respect and that he will be wasting his time contact you from now on.