We're all honest people here (I hope), and if it was just the one unexpected issue then I wouldn't rule out doing the honourable thing and make a small contribution.
But even if we give the buyer the benefit of the doubt, and we accept that this isn't a 'try-on' or a scam, I this it's obvious that in this particular case the buyer regrets buying the car and he's not going to be happy, whatever you do.
I therefore think that engaging with him further is pointless. Again, even if all his complaints are real - there's nothing you can do for him really (other than taking the car back).