I believe you and the Seller need to make your way to the NC DMV (must be a license plate issuance center):
“3.8 ASSIGNMENT OF SERIAL NUMBER FOR TRAILERS (MVR-15) G.S. 20-4.01, 39, 69
All trailers must be identified with serial numbers. Trailers manufactured by the factory are given an identifying number. Trailers without serial numbers cannot be registered unless:
1. Ownership is established and proper ID has been furnished as required by G.S. 20-52(A)
2. If an out-of-state registration is furnished showing homemade trailer without a serial number, an NCX number will be issued to title and license the vehicle.
3. The NCX number is stamped on the rear axle and on frame near towing attachment.
MVR-15 is an assigned trailer identification number. The person who built the trailer or to whom the trailer was sold, prior to obtaining a serial number, must title in their name before selling. The MVR-15 is used as the proof of ownership in lieu of regular title documents. It cannot be used by a purchaser attempting to obtain a new number because he does not have or cannot obtain the certificate of title.”
The above is excerpted from the Section 1, Chapter 3 page 10 of the Vehicle Registration Title Manual:
https://connect.ncdot.gov/business/dmv/dmv documents/nc title manual.pdf
The MVR-1 and MVR-15 are found @
https://www.ncdot.gov/dmv/forms/
As these things go, the specific DMV agent at the license center, can make the process easy or difficult. But per the above, “The MVR-15 is used as the proof of ownership in lieu of regular title documents. It cannot be used by a purchaser attempting to obtain a new number because he does not have or cannot obtain the certificate of title.” Hence the requirement to have the Seller present with you at the NC DMV office.
This process worked for me when I purchased a used SX230 and its non-titled trailer back in 2012.