When I was living in Canada I bought a repo boat out of Salem Oregon. The import was easy, way easier than I thought it was going to be.
As stated above the government has 3 things they are most concerned about:
1) VIN for the trailer. If you can get that info and call Canada Customs prior to getting to the boarder it will significantly decrease your time at the border.
2) GST - you will have to pay this at the boarder at the time of crossing. Note only cash or major credit card
3) Bill of Sales. Do them separate as someone above mentioned. Make sure they are signed by the seller and you.
You might get lucky enough to have the seller drop the price on the Bill of Sales a couple thousand to save you a few dollars in GST but mark my word you don't want to get caught because they add a stiff fine on top of it.
I just used my trailer plate from my other boat to pull the new one home. When I got to the border the lady asked me if I was importing a boat I said yes. She made me pull up and come inside with my paper work. I took it in, she did the trailer search and was simple because it had previously been searched as good. I paid my GST and was driving again in less than 10 mins.
Trailer inspected at Canadian Tire in Wetaskiwin for $0.00 and registered in AB same day.
You can pm me if you want anymore help with this but it is easier than I thought it would be.