Do not be intimidated....

Only point of law might be whether or not you misrepresented it in your ad. Did you claim "100% working" or "like new" or "mint condition" or "no leaks"?

If you misrepresented the truck in any SUBSTANTIAL way in your add then I think he may be able to cancel the sale... but as others have mentioned... how do you know the leak or break light problem didn't occur for the first time when he drove it several hundred miles home.

To me you have 2 options

1) Tell him to go ahead and sue you (in my vacation rentals, almost every time I keep a portion of someones security deposit because they damaged my property, they tend to threaten to sue me. I tell them to have their lawyer call me - haven't heard from one yet). Tell him since he is going to sue you, that you will not discuss the case any more with him until you are in front of a judge.

2) You can go above and beyond what you're legally required to do and buy the car back from him, as long as
a) He pays for any and all transfer taxes
b) The car is in exactly the same condition as it was when he picked it up
c) You deduct $500 from the sale price for the additional wear and tear he put on it

I would stick with option 1) myself. the whole private plane thing makes me think that this guy does not need your "charity" which is essentially what you'd be doing if you were buying the car back from him. I might go with option 2 if it was a single mom who was a waitress looking for a car to get back and forth from work, and realized that an old truck may not be so reliable.... in that case, buyers remorse, contract still binding, but being a compassionate human being would drive my decision.