I used to think that there were rules, with most boats selling for 10% to 15% less than the asking price, but I think that has changed a lot in recent years. There seems to be a lot more variation these days.
For one thing there are a lot of people entering the sport who are not very knowledgeable. They will often end up paying too much for a boat, dumping a lot of money
into fixing the boat the way they want it and expect to get every penny back out of the boat. These boats have unrealistic asking prices. Good brokers will try to get these owners to be more realistic, but I think that the problem is so endemic that even brokers are limited in how far down they are able to get the owners to price the boats because of the comp asking prices are inflated.
When the inventory of good clean boats was pretty small, owners generally got closer to their asking prices. These days these over priced boats will languish on the market for years at a time with the owner not budging or bearly budging on their asking price but eventually getting frustrated and these owners will often accept offers way below thier asking price.
By the same token, some of the more knowlgeable sailors and brokers take the time to research
actual sales prices of comprable models, actually visit sisterships for sale
to understand relative condition, and try to set their asking prices close to their bottom line sales price.
It is your job to try to figure out what the boat in question is worth in the market place and what it is worth to you, which may be more or less. I try to set an ideal price target and a max price that I will be willing to pay. And then I start the negotiations perhaps 5% below where I think that I ideally want to end up.
Owners generally assume that you will come up a bit and may counter offer somewhere between your offer and above where they want to end up. With a very over priced boat, I try to send a message that I am only willing to come up a small amount by countering the counteroffer with a small implemental increase.
Some owners will bail out of negotiations if my numbers are too far below their asking price, others owners will continue to negotiate if they are anxious to sell the boat and at that point you usually know where you stand. There will be times where you may need to write a position paper that shows the Seller that you know what you are doing and understand the marketplace and that you have a basis beyond low-balling or your own finiancial situation for the price range. When I have had a price limitation I may mention that limit to the buyer such as noting that my offer is based on the max value that I can borrow for that model plus the required downpayment and some of the other boats on the market that I can buy at that price.
It doesn't always work. Be prepared to move on or pay more than you want.