Buying US boat in Canada?

Hi Guys,

I am pretty clear on the rules of buying US boat from the US by driving down
south and towing it back to Canada.
But, I couldn't find any info on what happens if the US owner of US
registered boat is in Canada with his boat (towed by road) and decides to
sell it to Canadian resident while in Canada and goes home to the US without
his boat? Any formalities, how and what taxes need to be paid by the new
Canadian owner and will the old US owner be asked any questions at the
boarder when leaving without his boat?
Anyone can give some info/links on that case scenario?
Thanks.