I've sold boats to buyers in Canada twice, although not recently, it was within the past 10 years or so.
One was going to Ontario, and since my state does not title small trailers, the buyer opted to just buy and register a trailer in Canada and then come get the boat. I knocked a bit off the boat's price since I could sell the original trailer local.
The other was a guy who was returning to BC, he was here on vacation and found the boat and trailer I was selling on CL. Since that boat had a title from PA, he said it would be no big deal, just a bit of time and paperwork.
I'm not close to Canada, so its not a common occurrence here but I think its a matter of knowing what your province or Canada customs will be looking for.
I did buy a small, older boat up in Quebec once, but I had an trailer tag under the seat of my truck registered to a homemade trailer, I just swapped the sellers Quebec tag out for mine and came home with it.