Priceless mistake sold at a secret price.

The world's most expensive postage stamp: "Gul tre skilling banco." The multimillion dollar stamp from Sweden was recently sold at a record price to anonymous buyers. The 1857 "Three Shilling Yellow" stamp was a misprint that was made during a printing of eight shilling stamps that were yellow themselves. However, this stamp was supposed to be green — and has become a one of a kind rarity.  

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• The world's most expensive postage stamp is from Sweden, and it sold one more time last week in Geneva, Switzerland, this time again for a record high price. The 1857 "Three Shilling Yellow" stamp was a misprint that was made during a printing of eight shilling stamps that were yellow themselves. However, this stamp was supposed to be green. Now a one of a kind rarity, the stamp last sold for $4.5 million in 1996 at auction.

• This time, the David Feldman auctioneers would not disclose the identity of the buyers or the price, but they did say that it exceeded the previous purchase price paid back some 14 years ago. Speculators in the stamp world guessed that its previous owner was a principle in a Swiss bank who needed cash in the wake of the financial crisis to pay debts.
  The auctioneers did reveal that a consortium was behind the new ownership, and because it was considered a worthwhile investment, the previous purchase price has likely seen a healthy increase.

• The stamp was originally discovered by a Swedish youngster in 1885 and sold for small change. Since then, it has gradually increased in value while passing through the hands of some of the world's richest millionaires.