Every time I hear about "Rare Stamps", I am immediately repulsed! When it comes to Four-Letter-words, ya know, like that vulgar Anglo-Saxon term that everyone hates but uses so frequently ....
RARE, to me, is the vilest, most misleading, most misused word in the English philatelic language!
RARE, the adjective, is defined as "uncommon", "not widely known", or "seldom encountered". Yet I see thousands of stamps offered, every day, in online auctions, described as being RARE stamps!
Let's do some eBay searches! Right now, in the "stamps" category on eBay, there are 17,734 lots, with the word "Rare" in the lot description. Oh, my gosh! What a bonanza! These lots range in price from $0.11 to $55,000. A couple of the expensive ones are actually rare, like U.S. Scott # 121b, the 1869 30c definitive with inverted center, but most of them are not, and are described that way, in order to impress and / or mislead potential buyers.
Let's look at some scenarios here:
British Guiana, Scott #13, the One Penny Magenta is RARE. There is only one of them, and it is in private hands, so you CAN NOT BUY IT!
U.S., Scott #121b, the 1869 30c with inverted center, is RARE. There are a few of them in existence, but you would probably have a very difficult time locating one.
U.S., Scott #C3a, the Inverted Jenny. 100 were discovered, and all are now in private hands. You won't find one in the corner of your grandma's attic. This is a RARE stamp, as it would be very difficult to locate one for sale, even if you wanted to buy it.
U.S., Scott #245, the $5 Columbian commemorative, in mint condition. It is EXPENSIVE. Is it RARE? NO There are SEVEN of them on eBay right now.
U.S., Scott #C13-C15, the 1930 Graf Zeppelin airmail set. It can be EXPENSIVE. Is it RARE? NO There are 25 sets of them on eBay right now.
Switzerland, Scott # 3L1, the 1845 Basel Dove issue. It is VERY EXPENSIVE, cataloging 16,000 Euros. Is it RARE? NOT REALLY ... SCARCE MAYBE. There are two used ones on the German eBay right now, priced at under 2,500 Euros. There is a Swiss commercial website that I've seen, that has dozens of them, mint, used, and on cover! Also seems that just about every major European auction has a couple of them in it, as well.
Even stamps that catalog $25,000, if there are a lot of them around, are NOT RARE. On the other hand, a stamp that catalogs $10, if you can not find it, could be termed as RARE.
RARE should seldom, or let's say RARELY, be used in philatelic
descriptions. For most of the stamps, and really anything else, that
are offered as RARE, remember that this expression is usually a sales
gimmick to entice people to buy the product. The possibility that the
product is actually RARE is unlikely.