The us
customs is clear on requirements for importation of a vehicle (that includes trailers) from Canada to the us. You will need to fill out dot form hs-7 at the border. You will bring that stamped forum with bill of
sale and signed Canadian registration/title to your state licensing office & they will give you plates and mail you a
usa title for the trailer.
To meet the requirements of hs-7:
-Any trailer 25 years old or more that has
legal paperwork in Canada (registration/"title") will be no problem. You would only want to make sure the vin is marked on the trailer or have it engraved etc before showing up at the border (sometimes in Canada the info plate is only a sticker which easily falls off over the years).
-Any trailer newer than 25 years with Canadian registration/title would need to have the correct info plate, "this vehicle meets all us federal
motor vehicle
safety standards (fmvss) in effect at the time of manufacture" ... Some Canadian made trailers would have the correct info plate/sticker wording (tongue area typically), but many or most would not. "cmvss" is NOT equivalent to "fmvss" although the standards are probably very similar if not the same for all practical purposes on a boat trailer.
You can buy a custom engraved "fmvss"/vin info plate
cheap on ebay or amazon etc I believe, although I've never gone that
route.
I'm not sure about a trailer in Canada with no paperwork (perhaps pretty common for boats), I imagine there may be a
route to use receipts for materials, bill of
sale, and/or
inspection, etc to get a
legal Canadian
registration as a "homemade" trailer in some provinces. Of course that may not be a quick or easy process, but I'd imagine it's do-able in some provinces.
If the trailer has never been registered in the states previously, you may have to pay tax (6-8% or something) on the
purchase price (minus personal exemption if applicable). On the boat
purchase it's self, the duty is, I believe 1.5%. There may not be a title/registration for a recreational Canadian boat (for non
commercial boat bill of sale is all that's needed in some or most cases), but you will want a separate bill of sale for the boat & trailer clearly stating price for each. The usa
customs offers are confused by the Canadian transfer papers (bill of sale) so I'd suggest print/bring a few copies of bill of sale from your state.
You may find the customs officers themselves have varying degrees of leniency & understanding of the rules, but if your trailer
import falls within either of those categories above, it's an easy do it yourself process- no need for a registered importer or anything. If the trailer is under 25 years old with Canadian paperwork but no "fmvss" label, and you want to strictly play by the book, a registered importer might be able to "bring it up to compliance" and charge you quite a bit of money to get it across.