The older it gets, the more difficult it is to establish a market value.
I bought my 1992 SR 5.4 2010. Original tubes, was far from perfect but OK' ish. At that time my idea was to use it a few years and then re tube. Now done a few minimal DIY repairs, still don't need to re tube in a few years. In my case the value of the boat without engine is not that much but for me its still worth plenty as its usable without any investments.
I know this does not help but for really old boats the price is fair when buyer and seller are both happy...
fun on a boat is inversely proportional to size...sort of anyway