The hardest part is getting someone to contribute freely, at no compensation.
Dealers will make noise that prices are out of whack, too high or too low, but most of them contribute nothing.
I send out trends gathering information regularly, but to those that contribute nothing, it doesn't get sent again.
Public auctions are good for information, one needs to figure in the taxes and hammer fees without regard to who the buyer is. Ebay is one method of information, often with extremes of high/low over a short period of time, and unless the grade in known, much can be taken with a grain of salt.
Prices in trends attempt to reflect the market, with limited resources and often poor information. I need to round down prices when some dealers suddenly puch for higher prices as if it relects their stock or a recent purchase.
A comment from one person was that all the prices are fine makes me wonder if the person actually sells anything.
When a dealer is bidding above current trends or current book prices, it then stands to reason that that price is too low.
While paper is only a small part of the overall market, that market tends to be dominated by a handfull of large dealers, with many smaller dealers and eBay dealers taking up the slack.
If a 1937 $1 in Unc. regularly sells for $30, with highs of $40, lows of $20, then the trends would be $30. If the same note suddenly starts to sell for $35-$45, the trends would need to be increased.
If a dealer (who has a buy for these), starts to offer $50 each (and wants a limited number), trends would likely not reflect any changes unless the buy number is so high as to suck the market dry.
Some years ago, a large dealer in the west was buying Calgary Olympic sets for $500 each. The current market price was $275 per set. He made it very clear that when the order for 2000 sets was filled, he wanted no more. Trends did change for a brief period of time until the order was complete.
In the case of notes that are rare, the trends or book price merely reflect the information given. The price of a 1935 $25 note in VF or EF is how much? Perhaps the dealers and collectors who submitted prices never had one, never sold one, never seen one. Perhaps the one that contributed sold 4 of each and thus overwhelms the book price as the only major contributor. It is this sort of thing that makes it difficult to establish a base price for anything even slightly rare.
Rick