A recurrent complaint in the philately world is the language abuse of the word 'Rare'. How many times have we seen, especially in dealer's descriptions or on Ebay, the adjective 'rare' and all its declinations (very rare, extremely rare, rarity, etc.) pasted all over the place. Depending on whether one is selling or buying, the same item is either scarce, rare... or suddenly through God's grace becomes completely common. 'Anarchy' describes many of some seller's or owner's personal evaluations.
As an example: when I see some French dealers stubbornely describing a 15 centimes Cérès 1849 used as 'R' (cat. #2), while there are probably a few thousand copies existing, I can only disapprove such methods. Russian stamps have no better treatment.
The Cérès 1849 - 15 centimes green (cancelled) is often described as 'R' (rare) in many French auction catalogues.
I could easily find 100 copies to sell in a week, how can it be rare?
Let´s bear in mind the TWO factors determining rarity: how many items are known AND how often they are avalaible on the market.
And let´s bear in mind the gap existing between rarity and popularity (although the later can partially influence the former): how many people are willing to pay for an item, irrespective of its rarity.
Although no rating system can ever be qualified as perfect, and how could it be regarding the very complex nature of stamp collecting, I am in the strong favor of using a scarcity scale that would be both easy to use and reliable.
Besides reliability and easiness of use, this scarcity rating model should be easy to understand, easy to remember, and satisfy most of the advanced (read: knowledgeable) philatelists. I guess some of the criticism would come from the dealer's side. After all, describing anything slightly uncommon as being 'rare' makes it more likely to sell at a higher price, isn't it?
Some dealers though are making a good job and are using the adjectives scarce and rare only when necessary. Paul Buchsbayew of Cherrystone is rather cautious when describing some items' rarity; my colleague Terry Page, when selling his Zemstvo stamps on Ebay, also describes them with an excellent precision. Generally speaking (there might be some casual mistakes), all famous worldwide auction houses such as David Feldman and Harmers are doing a nice job. So everything is not so chaotic in our stamp world :-)
When selling my stamps through Ebay's Filarossia, or when writting down a description for an album's leaf, I inspire myself from the rating scale created by Carl Schmidt, the famous Russian Zemstvo specialist. It is an excellent system which has survived the test of time and was adopted by some great names like the Fabergés. This tells a lot about its reliability. This system could easily be used for any area of classical philately, be it Classic Greece, USA, Australia, etc
Here is the scarcity scale (the first three lines are my own):
Uncommon : 101 to 200 copies known
Scarce : 51 to 100 copies known
Very Scarce : 26 to 50 copies known
R = rare : 16 to 25 copies known
RR = very rare : 9 to 15 copies known
RRR = extremely rare : 4 to 8 copies known
RRRR = great rarity : 1 to 3 copies known
Of course, using such rating scale requires a deep knowledge of the subject at hand. But even, it is helpful in separating the wheat from the chaff: with this scale, a cancelled Russia #1 (with no special feature) simply cannot be described as rare!
Readers are welcome to share their insights on the matter. And it is fine to disagree.