In reply to Fozzy:
You can see it as "being an idiot" or you can see it as "doing the right thing". Personally, I think it's the latter.
If, as you assume, he's savvy enough to be expecting issues like that at the price then he won't much care when you tell him about them. If your argument is that he must be expecting it anyway then you've got nothing to lose by telling him.
If he's naive enough to think he's getting a brilliant car at a bargain price then you've bought yourself some good karma by clarifying for him. He might be impressed by your honesty anyway.
I know that's not necessarily the way most people expect sellers to behave but it would be a nicer world for everyone if they did.
I wouldn't go around a car I was selling pointing out every bump, scrape and imperfection. Those are clearly visible to anyone who cares to look, regardless of their expertise. I would point out any significant issues I'm aware of that aren't visible to inspection. The kind of things, like pissing oil and water, that you only discover over the course of time. Perhaps I'm being a complete fool but that just seems like common decency to me. (Though I wouldn't be naive enough to expect the same from anyone I bought from.)
It also saves you from having to worry about him hassling you later, as you currently seem to be. Perhaps your conscience already thinks you should be more forthcoming.
Post edited at 18:26