General Factors

• Many factors affect rarity, with age and condition leading the list. As a general rule, nearly all unused stamps of $1 or less printed in the last 60 years are only worth their face value or less. Used copies of a stamp are also worth far less than unused ones. One major exception are used stamps found still on envelopes, which should never be removed without consulting a collector.
  
  

History

• Stamps issued before 1925 and in mint or excellent condition bring the highest prices. So do stamps with unusual histories, such as the so-called "Confederate provisionals." Southern postmasters issued them after June 1, 1861, when the emerging Confederacy stopped using its Union enemy's stamps. The provisionals filled a need until October 14, 1861, when the government began printing its own stamps again. Thus, provisionals are worth $1,000 to $1,500, while other Confederate stamps are only of historical interest.
  
  

Other Relevant Factors

• Strong or unique cancellation marks can significantly impact a rare stamp's value.post stamps image by Svetlana Kashkina from Fotolia.com
  
  Stamps or collections certified by organizations like the American Philatelic Expertizing Service tend to carry higher values. While mint condition stamps are worth more money, collectors must take other unique factors into consideration. Limited and special commemorative editions elicit greater excitement, as do color intensity, location of image and the size or intensity of cancellation. Any or all of these factors can significantly elevate a stamp's desirability and value.
  
  

Errors and Flaws

• Stamps with obvious mistakes, such as reversed images, carry higher values.stamp image by CraterValley Photo from Fotolia.com
  
  Stamps featuring obvious errors and flaws have yielded some of the highest values. One of the most famous examples is one of Britain's earliest stamps, which appeared in 1840. Nicknamed "Two Penny Blue," these stamps omitted the issuing country's name and sold for $4 million in 1993. The world's most expensive rarity is the Treskilling Yellow, issued with the wrong color in 1855 and, more than a century later, valued at $2.5 million Swiss francs.
  
  

Expert Insight

• Short of attending a stamp show or consulting other collectors, a good visual inspection---with help from an illustrated guide---is often the best way of determining if rarities exist in a previously unseen collection. However, while a stamp may fall into a certain price range, its ultimate value depends on its condition, rarity and desirability to the collector. For this reason, it is impossible to place one set value on many collectible stamps.