Bobbie yes you are correct you need to give the seller a bill of sale -showing the price of the trailer seperate from the boat. As the trailer is more than 15 years old it just means the import process is even easier for the buyer. They do not have to pay about $200 to have the trailer registered into the RIV program at the Candian border and it saves the border staff from doing more paper work at the border. It also means they can get register and insurance for the trailer right away when they get home rather than waiting for the RIV paper work to come through.
My understanding/experince of the import system having imported from the US my car and trailer is that only motorized vechiles need to report to the US Customs prior to importing. I did have to send the paper work to US Customs ahead of time and then go in and pick it up at the US Customs office first once I had the car but I did not have to do either with the trailer - just showed up with the trailer & paper work at Canada Customs.
There is a good link to information on import boats to Canada: Pat's Boating in Canada: Importing a Boat from the U.S.A.