I am in the process of looking to purchase a boat also (38 to 42"). It is my first keel boat purchase but I am not an inexperienced negotiator.
1. There are many ways to say no, some no's are more offensive than others. Likewise there are many ways to low ball a purchase. Remove all personal emotion from any conversation to the seller broker or owner. Treat it as a business deal that you are out here to make money from other's boat.
2. Find out the background of the boat
a. The previous purchase price, number of years ownership, money spent on upgrade and repairs. Use these figure to find out his annual cost of owning the boat. The number should not include the slip fee and month to month overhead cost.
b. Reason for his sell, how urgent for him to unload his boat
c. how much monthly upkeep for him to keep the boat for 6 months or a year
d. Does he have montage on the boat?
3. Low ball him is OK, but show that you are ready to deal. Put up large cash escrow as a good will gesture to deal. Make him aware that this is a clean deal, you have no baggage. However, the deal is dependent on a clean survey and sea trial.
4. Previous sold prices are good to know, but it is just a reference. Use it to your advantage, but do not use it if not, since every boat is different, so are the buyer and seller.
Good luck to you and to "me" also. Remember it is just a toy no matter how we sweet talk our SO. No one dies of missing a deal on a boat. Hold on the your money, walk if you have to. There are more boats to come. The ocean will be still there when you are ready.