If it were me, I would insult the hell out of them with my offer based on that broker's comment.
Anytime you're dealing with the seller's broker, you have to remember that they're in it to get the most possible money for their client. And the most possible money for their commission. This broker is not working for you. Same as with real estate....use your own agent, not the seller's agent. If you're acting as your own agent, you've got your work cut out for you so you have to decide if it's worth the 5-10% commission fee you're saving.
The research advise is good, for your offer. When I bought my boat, I swear I think I looked at every C25 that was on the market at the time! I came up with a maximum market value of a perfect boat, average values depending on area, and low ball deals out there. When I focused on my boat, I inspected it very thoroughly (survey) so I had a laundry list of what all needed to be fixed. Not much, but enough to justify a lower price than asking.
In my written offer, I presented a summary of my findings. I compared this boat and asking price with other boats. I listed the repairs necessary to bring it to a condition comparable with other boats, and I listed repairs necessary to bring it to "perfect" condition, a sort of ceiling where no matter what you do, the boat will never be worth more than this. I made my offer of 58% of asking price, with documented reasoning of it.
I own the boat.
The seller told me he really wanted to get more for it, but was very impressed with the written offer and detailed information.....and all the research I had done. I told him I just wanted to present a reasonable, honest offer for what the boat is actually worth.