Use the search function drop down menu at the top of this site - use the bottom box (google) for best results. You will find lots of discussion on this topic.
The process and how much trouble you will or will not have depends greatly on the age of the trailer and/or if the trailer manufacture is still in biz and building trailers.
No Duty of any trailer going either way if it was made in Canada or the USA.
Sales Tax is set by the state you will be registering the trailer in.
Make sure when buying in Canada that you receive from the seller a signed copy of the Owners Registration
Transfer papers! Also make sure they are the person who is listed as the Registered owner on that paper work. Without that you will NEVER get it registered in your name.