Looks solid to me. Has the potential buyer seen it in person? If not, he really has no basis for saying that, at least based on those pictures. The only thing I see that is kind of iffy is the exhaust, but even that I'm sure is just surface rust. He's probably just trying to get you to come down on your price. If you have no problem selling it for a little less, throw him another number and see what he says.
