I have seen trades happen between private owners, but of course the stars must align just right, so it is a low-probability event.
I do not know about brokers, I have never heard that happening, but I would assume it is theoretically possible, although it is hard for me to understand what would it be in there for the broker to make such a transaction happen.
Boats are very different than cars, it is a relatively low volume business, and a broker does not own the used
boats on her
dock or yard or webpage, while a car dealer does own the used cars on her lot.