What is the year of mfg for the trailer? Does the title have a VIN?
Here is the link the importer should use to determine the required documentation and applicable fees along with any modifications that may be required by Transport Canada. Without the required paperwork there will be a question about the trailers admissibiltiy.
There should be no duty paid on a U.S. made trailer imported by an individual but the trailer will have to comply with road safety requirements. Read on.....