To find pricing for rare stamps, check the Scott Standard Postage Stamp Catalogues. This resource lists and values postage stamps and is available at most public libraries. Also, contact local stamp dealers and clubs for value estimations.
When pricing rare stamps, there are a variety of resources to consult. The American Philatelic Society, a nonprofit, member-supported organization, supports and promotes stamp collecting. The APS offers a selling/appraising pack, available through its website, containing guidelines, tips and a list of professional dealers. In order to ensure an accurate and fair price for rare stamps, consult more than one dealer. Consider visiting a stamp show, where many dealers could appraise your collection at one time. According to the APS, it is important to remember that old stamps are not necessarily valuable stamps. Many stamps dating as far back as the 1800s sell for the minimum Scott Catalogue value of 20 cents.
When appraising stamps, it is also important to distinguish between a revenue stamp and a postage stamp. Revenue stamps are generally found on documents. If you are unsure what kind of stamp you have, the APS suggests leaving the stamp on the document until you find out more about your materials.