Make your offer based on what you're willing to pay.
One example, for a well-used retired racing boat:
Asking Price $12,000
Sold for $8,000
Surveyor's Reported Estimate of New Replacement Cost $145,000
Surveyor's Reported Estimate of Market Value $20,000
Surveyor knew the agreed upon price and asked buyer what Market Value should be in his report. Buyer suggested a figure and surveyor concurred. Surveyor thought the seller set their asking price too low -- they should have started at $20 or $22K, and let the boat go for maybe $16K....
This of course is the opposite problem to what you're dealing with (seller setting asking price too high), but I guess the point is that it is a buyer's market and there are a lot of boats out there...