We recently bought a trawler and here are some things I found:
Several boats had past surveys and purchase paperwork just sitting and (seemingly
available for perusing. Several (in the 50 to 80K listing range) had about 30% subtracted from the agreed upon price after the survey.
The agreed upon price (original) was around 10% less than asking / listing price.
Several had obvious errors in the yw ads. I was able to talk with the current owner of one boat about it and the brokers just ball parked the details of a lot of things - ages of batteries, number of batteries, number (not size) of various tanks (holding, freshwater, fuel) and age of props.
All of the brokers except one were super nice, personable, funny with lots of quips about boat life - and they all wanted their money as quickly and easily as they could get it.
The exception was nice and also very honest, pointed out problems and made a point of saying which details he was sure about and which details were just conveyed to him from a seller or other broker - this is incredibly important!
From what we experienced and from what I learned looking through piles of paperwork it seems that (in the 50 to 80k range at least) that offering 10% below list price at the very least won't be offensive.
Mechanical issues and things below the waterline seemed to be discounted 100% from the agreed upon price. The surveyor made a list, the boatyard gave an estimate, the estimate cost was completely subtracted from the agreed upon price. This did NOT include cosmetic things for the most part.
Yacht brokers are just like used car salesmen. Some are good and some are bad. But all of them have a purpose and that is to make money and that money comes from you and the seller. At best a good broker wants that money as quickly and easily as possible. A bad broker will flat out lie, either by omission or just plain not telling the truth.
Ask the broker what happens if you find the ad is incorrect. Beware a broker that says "Your surveyor will find if there are any problems so don't worry about it now". They know that after spending 2k on a survey and haul out you are not likely to back out. So be prepared for that and be prepared to get an estimate for everything the surveyor finds.
Kind of unrelated to this particular issue but I've found that paying someone by the hour gets the best service, even from a surveyor. The hard part in anything is finding a good person to help you out
And for us, once it was all said and done the main thing was to focus on enjoying the boat and not fretting over if we paid too much (though I did file a complaint about the broker who said we had a spare never-connected water tank that turned out to be full of water and diesel