I agree that stamps shouldn't be added to Numista.
I say this as someone who is interested in "economic tokens" of all sorts not based on their shape, composition, or official/private status, but based on their function, so I see coins, notes, stamps, bus tickets/tokens, bread tokens, etc. as part of the same overall system. I guess I'm approaching it as if I were an anthropologist.
So, I do collect stamps, especially earlier (19th-century) issues, first and foremost Canada's, but stamps have become so cheap that I have been buying lots of different countries of interest to me (Australia before the centralization of postage in ca. 1910, French "classics", German states, etc.). It's interesting that a number of nations had stamps before they had their own coinage. Some colonies, too, never had their own coinages but did issue stamps, such as Vancouver Island/British Columbia.
Now, there are so many issues of stamps all the time that the number of coins being produced pale in comparison. Although stamps were first issued only in 1840,
Colnect has over 1,000,000 catalogue entries. (From what I understand it's the best site to enter one's collection and I may well go with it when I start cataloguing everything.)
I recently became aware ─as Idolenz points out─ that exonumia is already quite a challenge for our dedicated Numista referees. Yet the varieties that we encounter in coins, tokens, exonumia, etc. pale in comparison to the issue of cataloguing shades / re-entries / 13½ vs. 14½ perforations / thin vs. thick and laid vs. wove paper / etc. etc. etc.
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