OK, basically, I mean EXACTLY what I said, but there is some noisy static from somebody who thinks he knows better. Well, this is the voice of experience, so pay attention. I am NOT suggesting anything illegal, nor am I advising you to lie to Canada Customs.
When you bring the UNASSEMBLED trailer to the border, you are NOT importing a vehicle. You are importing a bunch of parts that will help you make a trailer when you get home. Tell Customs "I am importing some trailer parts." That is what you are doing, so say that. If asked how much you paid, tell the truth.
You do not need to present any papers at the border unless your declaration of the price is disputed, and then you show the cash register receipt, and NO OTHER PAPERS.
You do not need to pay any $200 RIV fee. That is crap.
I do not know the duty on hardware, but it is not severe. For me, it was waived.
I have crossed the Peace Bridge over 70 times, and never paid anything except the toll.
I have carried a Harbor Freight trailer kit into Canada on a trailer with an Ontario license.
I declared that I was importing about $140 worth of parts to help me make a trailer,
which was 100% true, because the wood to make the floor was later bought in Toronto.
The trailer was later licensed in Ontario, and still works fine. No papers went to the
Motor Vehicle Licensing office except a picture of the queen on a $20 bill, to which
two toonies were added to make $24 for the plate. Maybe prices are higher now.
Got it now? 100% experience talking. No lies, no distortions, no stupid fees.