Re: Purchased used trailer w/ no title
State laws vary. What works in FL may not work for you in another state.
The only thing you can do is go to a tag agency and try to get the trailer tag and boat put under your name. If they let you then great. If not then they will tell you what you are missing.
Then go to another tag agency - only this time tell them you made the trailer yourself.
And if that does not work, go to yet another tag agency with another story. Keep doing this till one of them does it. Then you are golden.