As I now have the paperwork in front of me that was required when I went thru the process I'll try to lay it out for you as best I can recall.
At first I tried contacting the DOT and a US Customs agent directly. The DOT was do it by the book. The Customs agent outcome was pretty much depending on who was "at guard" at the time would depend on how the process went. He highly recommended I "do it the right way", so I hired a Registered Importer and Customs Broker. I didn't want to invest the cash and 5 hour drive to the border to get rejected.
The RI met with the seller and verified that the trailer was DOT/NHTSA compliant and labeled it so with a sticker placed on the trailer.
The CB took care of the customs paperwork. I had to fill out a "combination invoice", "inward cargo manifest" and "customs power of attorney". After the paperwork went thru the bureaucracy I recieved a letter stating that the USDOT acknowledged the receipt of certificate of compliance for the trailer.
As an individual I found the process frustrating, but worth the cause. Heck, it's just a trailer, right? For those selling cross border goods all the time, I'm sure the process is much smoother and cheaper. If your seller is unfamiliar with the process than I'd suggest an RI.
Also, note that the seller towed the trailer across the border for me and I met up w/ him at the CB's US office. He was coming stateside that day and it saved me another 2 hour's drive. The seller had no trouble w/ Canadian customs w/ the paperwork my RI provided to him. As far as I can recall all he needed was the US paperwork and his Canadian receipt. If you are going across borders yourself I can't see anything changing, but the RI should direct you on that.
Hopefully that was helpful for you.
regards,
W