That's why I think it's fair to ask the seller to be as upfront as possible but I certainly understand avoiding anything that will mark the seller as having unrealistic expectations, if in fact they don't.
Everyone approaches negotiation from a different perspective. If you like a particular boat and the seller indicates motivation by cooperating with you in your legitimate quest for more relevant information (not a struggle for dominance) and by pre-approving the contract you intend to use, then go inspect the boat and make an offer for what you believe is a reasonable price. The worst thing that will happen is a counter or no answer. Some people believe in putting a large buffer in the price in anticipation of meeting in the middle.
(By the way, "Getting to Yes", by Ury and Fisher from the Harvard Negotiation Project is undoubtedly the best manual on understanding effective negotiation.)
I bought my boat after making a written offer on another boat that was declined by the seller with a minimal movement counter. I had inspected the previous boat and took a test sail with the owner, who seemed to be a nice, decent person. The whole process took up a day of my life, but it was overall pleasant. I liked the boat and even imagined how I would sail it home, but we could not agree on price and terms. I did not spend any money other than gas money to drive to see the boat and lunch on the road.
I agree with GeorgeB - start making some offers. You are overanalyzing the boat buying process! Don't get hung up on a particular make or model or attached to the outcome, think about your purposes. Stop worrying about the brokers, who should be the least of your concerns. While you have been talking about buying about the last 2 years, the rest of us have been sailing...