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When I bought Berg's trailer in NH, I used his plate to get it down to MA, stopped at an rmv on the way home, put the new plates on, and mailed his back to him. Assuming the guy you're buying it from has a current registration, probably the simplest and least risk to get it done.
As insurance coverages for trailers are attached to the vehicle towing, if you did happen to have an incident in transit, you were borrowing your friend's trailer. No effect on his insurance.