That's like negotiating best price on a new car and "then" asking how much for my trade in...
Um, sorry, but this is the RIGHT way to buy a car... They can offer you squat for your trade, or no change on MSRP, but if you negotiate the price of the car, then you see right away where they are making their money if they offer a terrible trade in price.
You either A) haggle the price of the new car, THEN B) haggle the trade in, or you haggle for a combined number that the salespeople would rather do so they can BURRY THE BUYER with numbers (this is a VERY old car sales tactic, and if they pull it on me I walk).
Nothing wrong with offering to buy the boat for X, assuming it in as good of condition as it "appears," by his non-expert opinion. If he then surveys the boat (at HIS expense), and finds NOPE that pristine motor is blown, then guess what, it was a "SURPRISE" he should offer a lower price based on that.
How is this any different than offering a price on a house, acceptance of the offer, then inspection of said house, and the inspector says GUESS WHAT there are termites? You telling me he's bound by his offer? for something he couldn't see? What's the point of an inspector again?