Last month I bought a boat and trailer from a private seller in DE and just registered in NJ.
I paid around 3.5% for the boat and 7% for the trailer to NJ DMV.
The boat didn’t have a title, DE doesn’t title boats, so I needed a notarized bill of sale, the previous registration, a picture of the boat and pencil rubbing of the HIN.
The trailer was titled so a seller signature and price on the title is all I needed.
If the trailer is not titled make sure you get a seperate bill of sale. NJ wants to see a price paid to charge you appropriate tax.
It took me three trips to the DMV to get all this info a little at a time.
Hope this helps.