I have been a boat enthusiast all my life so messing around on boats as a broker for five years was a natural for me. I have been away from it for about three years. Like any other professional or near-professional, your experience will vary tremendously. A good one can help in several ways. First off, they help keep buyer and seller from getting into a shouting match. The broker does have an ulterior motive to close a deal, so will do his/her best to get questions answered and an agreement reached that both can live with.
A better broker will get to know you and help you reach a decision that makes sense for you. I have actually tried to talk people out of buying a boat, especially when they had virtually no experience sailing and wanted to start with a long voyage. I probably lost a few sales, but my conscience is clear.
A really good broker will understand what you REALLY want and will help show you why a particular boat will suit your REAL needs better than another. By the time we bought our current boat, a broker was almost irrelevant because we knew our minds after a getting a lot of experience with boats over the years. Still, our brokerage knew a lot more about the details of our boat than we did. The brokerage was not the only source, but it was the most efficient.
A brokerage routinely deals with the administrivia of buying a boat and will help you keep from getting too starry-eyed at sea trial and survey time. They do a lot of the routine work for you.
All of that said, if you really know what you're doing and can find a good private sale, buyer and seller can avoid fees.
I'll use a broker when we sell our boat. Couldn't imagine the hassle of selling it myself! Besides, exposure is better. High end boats need a broker. There's not enough money to justify the effort for boats under maybe $15k for a brokerage to get very interested.
Just a few thoughts.
Where's Bernie?