I just picked up a used boat out of state and my DMV said the boat needed to be registered.
My options (told to me from DMV) were
1. ask to use the current owners plates until I got home then mail them back.
2. Go to the DMV in the state where I was buying the boat with Bill of Sale and get 30 day temp tags
3. Get my plates before I went. You and current owner could do bill of sale via fax and that would work to get plates.
I ended up getting a new trailer at last minute as the one with the boat was not rated for enough weight, so the dealer got me set up with temp plates.
I think a bill of sale would only work if the boat was still registered in the sellers name. If it is not registered to anyone I don't think it can be driven down the road legally. Just my thoughts
Brian