I think so long as you call them ahead of time and provide the proper/required documentation prior to showing up at the border, its no problem.
I've never done it myself, but i've sold several vehicles to Canadians.. many had no more documentation than a hand written bill of sale, and right thru the border they went.
Might be different if you're hauling brand new Farraris or BMW's or something.. but old cars & trucks (30+ year old ones, anyway) don't seem to be much of a problem at all.. at least not going UP anyway.
Coming from Canada down might be more of a hassle.