And, as for needing a broker....
Well, if you've NEVER bought or sold anything and have no clue, there's an argument to hire a broker, at least for the paperwork.
However, it's not hard, but one needs to know some things and how property is transferred. You can usually get the legal way to sell a boat pretty easy from the state or CG if documented. Buying a boat and registering it is a tad harder, but also can get info from the state or CG.
The issues come up when there's a problem, disagreement, misrepresentation (real or not). So, there's an argument to have a sales agreement with the buyer/seller to protect one's interest. There is no standard and you can make up your own.
Also, you need a way to handle deposits (if needed), and that can be a hornets mess. Personally, I ALWAYS control my dollars or I just won't do the deal, and never had a problem. As a buyer, I'll only make a deposit with MY guy and if for any reason I don't buy, I get 100% back. As a seller, I rarely ask for a deposit, but if the buyer wants it off the market for more than a few days, I'll want something so I don't miss another sale should he back out.