Re: Looking for some perspective
From the story what I see (IAAL) is that there were a series of offers made by either side, but as of yet no acceptance of those offers, just a series of counter-offers. So as far as I can tell there is no contract, and I am not sure what you could sue based on.
The fact that the broker put a 'sale pending' label on the boat without a signed purchase contract to me indicates a broker that is actually going above and beyond what is required. Likely based on the fact that he believed the deal was going to happen, and didn't want to stir the pot with competing offers.
You may be miffed at this last minute offer, and I would be too. But I can't see anything that looks unethical or against the law to me. As others mentioned, most brokers are legally required to bring any offer to the seller, at any stage in the negotiations.