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Bob Rousseauthe cards get a premium because of the people trying to build the sets. In that sense, though it may be a common, to the people trying to get as many as possible, they're just about 'one of a kind'.
I bid on some RPPC postcards taken in photo booths in the 10s and 20s- and while most people wouldn't be interested (thankfully), I realize just about every time I'm bidding on a one of a kind thing. Like Jerry Garcia said about Grateful Dead fans "Our fans are like people who like licorice- not everyone likes licorice, but the ones that DO like it REALLY like it."