For starters, the guy's being an a**. I'm going to feel comfortable in assuming that you didn't say/write anything inflammatory to warrant his reactions in those e-mails. If he's going to go through e-bay for fraud, I would say you have even better evidence of harassment by him. His e-mails are over the line.
The most important things that he said were in that second e-mail that you posted. For one, he noticed the possible leak in the pics. He even asked about it. You told him that you knew of no problems. I don't think that your intent was to mislead him. You simply didn't know. If I were putting out the $$$ for the car, and that pic threw up a 'red flag' for me then I would want to check it out or have it inspected. He's mad at himself for going against his own better judgment, and not checking it out.
He said it himself THREE TIMES: "I took you at your word." Well, that was his mistake, and he's just p.o.'d about it. He had the option of doing more than just accepting your word. HE OPTED OUT. End of discussion.
I can appreciate your wanting to help a guy out. But, the car was sold as-is. I don't think you should spend a single penny.
He implied that you didn't do the work, didn't know much about the car, and (seemingly, by default) that he knows more than you. Fine. Then, he should have decided to bring his all-knowing a** to look at the car before buying. He's trying to play on your emotions and conscience by saying you had bad intentions, when you did not.
I've had several different vintage musclecars. And, on my budget, all of them were project cars that needed plenty of TLC. As others have said, when you buy a 40-year-old car, you're not getting showroom new or warranties.
Finally, FWIW, I am speaking from experience. I bought my wife's car on e-bay nearly 5 years ago. Even though it had been inspected, I personally drove and looked at it myself before committing to buy. I live in Louisville, KY. The car was in a suburb outside of Chicago. I took a day off of work, and hit the road early for a 6-hour trip each way. Left my house at 7 a.m. and got back home at 4 a.m. the next day and went to work. I know that our world is getting smaller w/ the internet, but some things still require you to put your hands and eyes on them in person before you can be as certain as possible that they are right for you.
Sorry about the long post and rant, but I really feel that this guy is being a jerk and trying to take advantage of you. Speaking to a lawyer is a good thought, if it eases your mind. But, is is "buyer beware" in this market. I don't think he has a legal leg to stand on. You had no ill intentions, and he's just trying to sleep better at night by making you feel like you did.
Good luck. Peace.