I've never bought a motor vehicle from the US but a year or so back throngs of canuks were heading south to by new and used cars and trucks because it was a lot cheaper. The one that got me was skidoos, made here in Canada but thousands of dollars cheaper to buy across the border. Go figure.
I think the buyer will need to check with the MVD on this side to get the lowdown. Anything over 35 years qualifies as an antique so is duty free. It will be a whole lot easier for the buyer when he goes to register it if you have registration papers along with a bill of sale.