Juanito I think returning it to Canada and selling it there is in your best interest (higher sales value in Canada) as well as that of the buyer (they will be able to take legal ownership of it)
The bottom line to selling a Canadian registered trailer or car in the USA can be found at: USA Customs and Border Protection - Requirements for Importing a Vehicle.
It reads in part:
"It is illegal to bring a car into the U.S. and sell it without first entering it through CBP. If you purchase a vehicle that was brought into the U.S. and sold without being properly entered through CBP, that vehicle is subject to seizure.
NEVER purchase a used vehicle in the U.S. if the owner cannot show that it is currently registered in the U.S. and demonstrate that it conforms to DOT and EPA standards."
BTW although the above refers to cars the rules for the importing of cars is pretty well the same for trailers.