The stamp collection you inherited from your grandfather may have more than just sentimental value. Old postage stamps are a big business. For example, the world's most expensive stamp-the 1855 Swedish "Treskilling Yellow" with a misprint-sold for about £1.5 million in 1996. It sold again in 2010 for an undisclosed amount. Although that's an extreme case, stamps often sell for four, five and six figures. It's worth finding out a stamp's value, because you never know when a hidden gem will pop up. The keys to a valuable stamp are appearance, condition and scarcity, according to the American Philatelic Foundation.
Have your stamps appraised. The American Philatelic Foundation, a non-profit organisation, has an appraisal service that determines the prices of stamps. Stamp dealers also perform appraisals.
Look at price guides. Scott Publishing Company publishes annual guides with prices of United States, United Nations and international stamps. The House of Collectibles publishes "The Official Blackbook Price Guide to United States Postage Stamps."
Check out the market value of stamps. Sometimes, there is a difference between a stamp's list price and a dealer's asking price. The dealer may think that a stamp is undervalued and offers it for more money. Or, the public can think a stamp is overvalued and customers won't pay book price, thus driving the price down. A publication such as "Linn's Stamp News" has advertisements from dealers and auction houses.
Stamps are worth less if they have creases, tears and are off-centred. Companies grade a stamp's condition on a 15-tier system ranging from Gem, a perfectly centred stamp, to Poor, the lowest grade.
Look out for stamp counterfeits. Insist on having the stamp professionally authenticated by an outside company.