Tell him to go eff himself. Used car sales are "As is". Even as far as suing, the only leg he would have to strand on, is if he could prove intentional fraud on your part- i.e. that you were aware of major damage and lied about it, or purposely concealed it (which I'm sure is not the case)- and he would have to PROVE it.
Probably just some flake who never dealt with old cars before and was unrealistic about what to expect. People see things in magazines and on TV, and think that every old car is like the $70K creampuff they saw there....and even if they got THAT car, they'd still complain, because in reality, they're still just old cars, even under all the shiny paint.
Just ignore him and block his emails/messages. Special Snowflake, who can't take responsibility for his own actions and even ask some questions- much less go and inspect (or have inspected on his behalf) will get tired after a while and go away, and go back to playing video games in his mommy's basement.
