If you remember having the title....
If you remember having the title and registration, and if the trailer had plates in your name, a duplicate title should be no problem to obtain. If you never had a title be prepared to give the new owner a notarized bill of sale. In any case, prepare a bill of sale and make 2 copies. On the bill of sale have the name of the buyer & seller, the serial number, the true amount of purchase, and the date and time of the sale. Sign both copies in the presence of the new owner and have them do the same. This will protect you from any dispute concerning amount of sale price and liability (should the new owner become involved in an accident later that day). Unless you have a personal relationship established, never take a negotiable form of payment (personal check, cashiers check, money order, etc.) For a large dollar transaction, try to do it at your bank. If you can't, use a counterfeit marking pen to check large bills.