My opinion is to return the trailer and get your money back. If the trailer originally had a title it's up to the previous owner to supply you with the title he lost. He can apply through the local DMV or whoever handles that in your State for one which may take a month to obtain. You probably shouldn't have paid in full without the title. If he fails to obtain the replacement title or return your money you will need to get a lawyer involved. You may like the trailer but without the title its not legally yours.