First - Call your DMV before you buy to be sure what paperwork they require.
You most likely will want a good Boat Trailer Bill Of Sale if you don't want problems later on or if the time comes that you want to re-sell it.
Receipt
This form of receipt should clearly state the details of the trailer that you are buying. Do not use a sales slip or receipt without all of the proper information included.
Selling It
If you sell it, you should use a similar form to make sure that both parties understand the deal so that there won't be any issues later.
The Boat Trailer Bill of Sale should mention year built, color, make, model, number, body style, VIN, length, width, number of axles, and GVWR.
The states aren't unified on trailer registration and title requirements. Some need a title, as well as bill of sale, and the rules vary as far as weight is concerned.
If the GVWR is under a certain weight in some states you do not need a title, but in others you do. Then there is the age.
If it is older in some cases, it doesn't need a title. In some states, you can register a trailer as homemade if, in fact, it is made from parts and that doesn't need a title.
Boat Trailer
Here is a very good Champion Boat Trailer.
So, the thing here is that you have to understand your title requirements for your state before you pay cash for something that will become a real nightmare in the end of the deal.