Before leaving to get glider, arrange insurance effective the day money and documents change hands.

It's generally easier getting a glider and trailer into the US from Canada than the other way around. But don't borrow a plate from your buddies. I've seen customs check VIN against plate – and their disappointment when they matched.

One nasty gotcha is radium dialed instruments. Get rid of them BEFORE crossing into the US. Those big yellow vertical panels at land crossings are radiation detectors. Free disposal in Canada:
https://www.cnl.ca/en/home/environme...-programs.aspx

Expensive in the US.

Check with your local FSDO and glider A&P what they want to see. Export CofA either direction for a used glider is likely a waste of time and money.

Every state has its own requirements for trailers. You might want to consider registering the trailer in Montana,especially if you are passing through..

Personal property tax is different in every state. Check with local glider owners.

Yes, separate bills of sale for glider and trailer. Temporary plates for trailer are usually easy to obtain. The seller should accompany you to the provincial licensing office.

As in the US the CofR must be returned, in Canada's case to the regional Transport Canada office. Take the CofR to the post office and ExpressPost it to TC along with statement that registration letters removed and code removed from transponder - they now charge $200.
https://www.tc.gc.ca/en/services/avi...-aircraft.html

A Canadian seller cannot take the glider across the border without customs broker paperwork. He might get hit with a $10,000 fine.