Your best source of information would be to contact John Marples. He sold them the plans and is probably familiar with the boat and situation. This boat's been listed for a long time now. If you are not familiar with wood composite boatbuilding having a surveyor
check the construction quality would be a logical step in applying a value. In truth a unfinished boat is hard to sell and is usually valued at the second hand price
of the gear
going with it. Owners aren't happy about this after the time and money
put in but that is usually the case. I'm concerned about the wording of the ad where it is up to the buyer to verify that the components of the boat listed as part of its value are still there.
For comparison check into the assembled Searunner
37s in Mexico
on Yachtworld. One for the same price
of 19,000 and the other for 29,000.