I believe that anyone considering the purchase of a used boat should have as much information about the purchase as possible. It is another data point that should help any prospective buyer make an informed decision. Knowing the market for a particular year make and model can be as important as knowing what brand of engine, or the moisture reading of the hull and deck.
I have to respectfully disagree with Main Sail on his characterization on NADA as "Their values have been about as accurate as a drunk shooting darts from 40 yards." Unless he knows better drunk dart players than I do...

NADA, IMHO, is
consistently at the low end of the spectrum.
I also, respectfully, disagree about soldboats.com being the ONLY close to accurate resource. BUCvalue (
Boat Values, Prices, Evaluations, Used Boat Price Guides - BUCValu) is another resource, which I believe is slightly more accurate, and it allows 3 price views in a 3 month time window. I have also looked at SOLD prices in eBay as a data point.
Using your example here is a comparison of the values from the three main sources (BUCvalu, NADAguides, and SoldBoats.com) for a '75 Columbia 26.
BUCvalu; $5450-$6250
NADAguides $5650-$6400 - Note that this is a little HIGHER than BUC
SoldBoats.com
Note the range of years (1972-1978), and the range of dates of sale (1996-2010) that I used in my search... If I tighten up the dates of sale to last year only, there were 2 that sold. The one in GA, listed for $2995, which sold for $2400 after 14 months, and one in NC, listed for $10, and sold for $10 after 20 months. If you were using SoldBoats to figure the
current average selling price of this boat (2410 / 2 = $1205), you would be far under what BUC and NADA provide.
I would state that there is more
information available through Soldboats.com. However, the only accurate measure of the value of a specific boat to you is what you are willing to pay for it, and what the seller is willing to accept.
I've said this elsewhere in SailNet; "The value of any specific boat (or anything) is only determined after a specific seller and a specific buyer agree to a specific price at a specific time. Changing any one of these can affect the price dramatically."
I hope this helps!