It depends on what kind of an agreement the seller has with the broker... If he has an exclusive contract with the broker.. he has to refer you to the broker... Some brokers allow "participation" with another broker. Some want to handle the sale exclusively. If the boat has been advertised for months... there probably is a time limit on the listing... Usually it is 3 to 6 months. If the broker doesn't sell the boat in say... three months... then he loses the listing.. or possibly he no longer has exclusive rights to the sale... The owner may at that time then advertise and attempt to sell the boat on his own, even when the broker is still trying to sell it. Whoever sells it first gets the commission dough. These are all terms that can be negotiated at the time of the listing. The amount the broker gets varies also... on small sales they usually want 10 percent... on larger boats they will take less of a percentage. All these things are negotiable.
I would approach the owner first and find out what he will do.... He may knock ten percent off because he is handling the sale and not the broker. He may also wait for an agreement to run out before selling the boat to you. This is kind of dirty dealing, but if the broker is sitting on his ass and not advertising the boat... then why should he get anything?