To alleviate issues in the future I suggest to everyone to have the bill of sale signed in front of a notary. Besides Buc Kelly Blue Books has one for boats but Buc is better to use. Each state is different as to regulations. However a notarized bill of sale helps to resolve many issues.
As rgranger pointed out when there is no documentation it is difficult and TN is another state not requiring registration.
To get around that as dealer a notarized bill of sale was required from the seller to new buyer. As a broker, I provided my brokerage agreement plus notarized power of attorney. Any addl info on trailer was requested for example tax documents, previous bill of sale, insurance policy with trailer noted and so on from the seller. With all that in VA and NC you can apply for a new VIN NUMBER which then is stamped into the trailer frame. The new owner then apply for a title and registration. Worked every time for me