Scarcity is a reason but not the only reason why an 1856 1¢ stamp is about to sell for who knows how many millions of dollars. The opening of an explanation from the Economist:

“ON JUNE 17th the British Guiana One-Cent Magenta stamp, the only surviving example of a penny issue printed in the South American British colony in 1856, will go under the hammer at Sotheby’s, a large auction house, in New York. Each of the three times it has previously been sold at auction it has set a new record for a single stamp, and the expectation this time is that it will do so again by a very wide margin. Pre-auction estimates are that it will fetch in the region of $20m, almost ten times as much as the price achieved by the current record-holder, the Treskilling Yellow, in 1996. That would make it the most valuable human artefact (by weight) ever sold, says the auction house. Why have prices for very rare stamps risen so high?

The broad reason is that number of extremely wealthy people in the world has soared in recent years.”