As I think you are aware, you are trying to close the door after the cows got out.
First step: Check with the govt office. Explain the situation and ask what you can do. My bet is they will say they can't give you title without the owner (on the title currently) signing off.
Step two: Go back to the seller and see if he has the original owner's (the one on the title) information.
Problem is you are likely to run into a wall where there is no good solution if the original owner can't be located or isn't cooperative.
A homemade trailer title works great...until you get pulled over and it clearly isn't homemade.