I agree completely with Mike O; the huge differences in condition and features among the same model boat, the variations in suitability for particular locations, as well as the miniscule number of transactions, makes the idea of true market prices for used boats a misnomer. It's more like buying a house than a car.
That being said, things (including boats) are worth whatever someone is willing to pay, and being social creatures, boaters are always looking at what someone else has paid for the "same thing". Regardless of whether they really are the same or are only vaguely similar. I've cut and pasted the valuation section of my pre-purchase survey below (sorry, but the formatting got screwed up in the process). Given my extensive research prior to making an offer, as well as my own general knowledge of used boats, I knew the sale price we had agreed to was fair before I saw this survey, but it was nice to see it in print anyway. Either way, I have been very happy with my purchase, and I know the PO was too.
VALUATION Method of Appraisal: The Business Method was not used because the vessel is used only for recreational purposes. The Cost Method was not used as there were sufficient recent sales to determine Current Market Value using the Market Method of appraisal. The following sources were used: YachtWorld and Sailboatlistings.com lists nine Catalina 28s for sale, built between 1991 and 1995, at prices ranging from $14,900 for a 1991 model to $26,800 for a 1993 model. The average asking price is $20,600. soldboats.com lists twenty sales of such boats within the past 24 months at prices ranging from $10,500 for 1991 model to $28,900 for another 1991 model. The average selling price was $22,200. BUCValuPro estimates the current retail value range of a 1991 Catalina 28 with a fin keel in BUC condition in the North Atlantic at $17,400-$19,800. ABOS Blue Book estimates the retail value at $14,613. NADA estimates the average retail, as equipped, at $19,800 Analysis: Discounting the average asking price by 15% suggests an average selling price of $17,500. The actual average selling price was higher at $22,200. The average of the three blue book valuations was $17,600. Based on the above data, I estimate the current retail value of a 1991 Catalina 28 in average condition at $19,000. I estimate the current retail value of this boat at $19,000. REPLACEMENT VALUE BUCValuPro estimates the replacement value of the base boat at $109,000.