A trailer is a registered vehicle with a VIN and a title. If he can't produce the title you don't know if there is a lien on it or if he even owns it in the first place. If you gave him the money and he then failed to produce the title, the police may intervene and find out if he is the owner. I can't tell you what the outcome will be or if they will help. The police will not get into civil matters so if you want their help it must be on a legal level. No title, no sale.
You don't have to show them anything except maybe the bill of sale he gave you. You asked for the title and he would not produce it. That's it. You want your money back.

