Thank you to everyone for your honest and intelligent comments.
As human beings we are great at learning the context of a social and economic interactions.
The business of collectible stamps is an intriguing one in that it seems to sit (perhaps uneasily) between being individual and unique items, and of being a commodity. The advent of the stamp catalog seems to have cemented the stamp business into this conjunction of two business models.
If the catalog says all stamps of this type and condition are worth $X, this has the effect of "commoditising" stamps. An example of a product being a commodity is to say one pure gold bar of a given weight has just the same value of another pure gold bar of the same weight.
Where the rubber meets the road though, we find stamps in a wide range of conditions, and grading a stamp is based on not just one factor, but on several (centering, sheet position, freshness, integrity, gum, cancel/markings, genuineness, etc).
Now I am not saying we should do away with stamp catalogs. They perform a very useful function exactly as a guide (as many have mentioned). Dealers rely more on the catalog than do collectors as they have to evaluate every item in their inventory to determine the right price to sell it for.
We collectors have the leisure of choosing to focus our attention on a single or select group of countries.
The fact is, whether we say, "Catalog value is just a guide," or "Stamps sell for less than their catalog value", is that the catalog value, while useful as a guide, for the vast majority of stamps, is not correct when it comes to the market value of a stamp even when the stamp is VF Mint NH and faultless. I think I can safely say that, generally, the listed catalog value is significantly higher than the market value of a given stamp.
This comes as a surprise to almost nobody on SCF, no doubt. This is the way the stamp world has been for over a hundred years.
It is changing, though, and Lorddenning has given us an example of how it is changing, through analysis of publicly available information about stamp sales.
revcollector and perhaps others said it is not wrong to quote the catalog value by saying, "catalog value is $X," or something similar. I would only say that while only offering a phrase or a few words, what you have at the heart of that statement is a prepositional phrase which in its full form looks something like this:
The catalog value of X is $X.
Or in a particular example: The catalog value of this stamp is $32.50.
While we may not say it like that, our minds understand the phrase to be the sentence that it stands for. In that case, while we may not be actually lying, it may be considered to be misleading.
Keep in mind, I am not personally against using the catalog value in describing an item for sale. I think Doug's post comes closest to how I think catalog value could be used in offering an item for sale.
I am in the process of trying to introduce a friend to the hobby. One of the first things I told him was to not make purchases on
ebay until he got some more experience under his belt. It was in thinking of how to introduce someone to the hobby that many of my own questions came up, and wondering why our philatelic world is so complicated. But perhaps if it wasn't it wouldn't be as much fun.