Only five copies of the 2001 stamp are known to exist.
Both the seller and the buyer were anonymous.
The stamp is especially sought-after by collectors. Hepburn's son, Sean Ferrer, ordered that the print run of the stamp be destroyed.
He objected to the cigarette holder dangling from the Breakfast at Tiffany's star's mouth and refused to grant copyright.
But 30 proof copies apparently escaped incineration. Most were likely binned, but a few have ended up in collectors' circles.
The stamp's resulting rarity - and massive appreciation since 2001 - have made it a holy grail for collectors.
Hepburn died from colon cancer in 1993.Mr Ferrer said he hoped that the stamp seller would donate his windfall to benefit cancer research or an anti-smoking campaign.