"Even if selling prices are reported it is difficult to report condition and equipment on sold boats." BUC does describe value expressly, and they indicate the value of boats and the adjustment for conditions. If, of course, you believe that they are reporting actual sale values. There's always been some speculation about that.
The sparsity of resales is something else again. If you are looking for a Rarity47.5 and there were only two sold in the past five years...yeah, you're gonna pay what the seller asks. That's the nature of market pricing.
There are plenty of boats that sit unsold for more than a year, two years, because the seller has an unreal belief in what the boat is worth. And the broker (often getting paid yard rent anyway) isn't always anxious to disillusion them.
I also follow the concept that you take a boat in whatever shape it is in, add whatever parts/time/labor it would need to put it in Bristol condition, and then subtract that from the top used boat value to get what it is worth. "Well it only needs a paint job" Yeah, sure, but if a paint job costs $25,000...that should affect value, no matter how willing I am to buy a dull chalky boat.