I've got to agree with much but not all that has been said here already;
1) The broker's overhead is his office, phones, advertising fees, etc. He incurs these costs whether he brings the buyer onboard or you do. For that reason he expects his fees to be paid whether he sells the boat to Joe Sixpack or your brother-in-law. If you think there is a chance that you may have a buyer tell that to the broker beforehand and have that buyer named as an exclusion to the contract should that buyer actually buy. The broker will likely want a time limit on the exclusion.
2) The contracts are pretty standard. If you want something changed, added or deleted you'll need to negotiate that before you sign.
3) There are many more boats than buyers out there right now and with fuel prices rising again the picture for sellers and brokers will not get better. If you have a desirable boat to offer the broker will be more likely to negotiate his terms. If you have just another "fill in the blank" boat the broker will put it in his listings but he knows its a slow sell so he won't be excited about taking any cuts on his terms.
4) I don't bother asking brokers how they plan to market the boat. I'd rather do my own research. Does he have ads in the local and appropriate publications? Is he on YW? Does he have a website? Are his ads complete, describe his offerings clearly and include photos? Is his office located near your boat or will he have to drive an hour to show it? Make up reasons to call him on the phone and see if he actually answers or do you always get an answering machine? How quickly does he call back? Get references!!!!
All of the above does not guarentee a good experience with a broker. When we sold the Carver we listed with the guy who had the biggests ads in the local publications. It took 18 months to sell it and all during that time the broker keep claiming he had one then another buyer "real close" to buying. Eventually another broker found a buyer. When we were considering selling the Silverton we spoke with the broker at our marina who seemed like a nice guy but we were told was not very aggressive. He had a showing before we even signed the contract, had a signed agreement within 3 days and stuck by the boat all through sea trial, survey and the adjustments we had to make. Even at that when the buyer wanted late concessions the broker took the extra $1K out of his commission.
So, two boats sold in the same season by 2 different brokers. Was the difference the brokers or the boats? I think it was a combination of both.
BTW, remember, the broker has a family to feed and bills to pay. Pick a good one and you'll be happy to pay him for his services. Pick a bad one and you're in for a long nightmare!