The value of a U.S. postage stamp depends on its degree of rarity compared to the demand from collectors, rarity in turn being affected by factors such as age, denomination, printing errors and condition. Each of these factors is looked at below.
Although remarkable numbers of old stamps survive considering their frailty, very early examples are much more rare and desirable than later ones. The first stamps issued nationwide in the U.S. were the 5 cent Benjamin Franklin and the 10 cent George Washington, both issued in 1847. As of 2011, the Stanley Gibbons guidebook values for this very collectable duo are £3,900 and £13,000 respectively. Prior to 1857, U..S stamps didn't have perforated edges, being snipped or torn out of a sheet instead. This is an easy way to spot valuable early rarities.
The denomination of a stamp is the value printed on its face. This affects rarity because at any given time there are far fewer high denomination stamps in circulation than low denomination ones. This explains why the 10 cent George Washington stamp mentioned above is worth over twice as much as the 5 cent Benjamin Franklin. Similarly, the 12 cent black of 1851 has a guidebook price of £2,600 according to Stanley Gibbons as opposed to £260 for the 1 cent blue. When looking for high denomination old U.S. stamps, bear in mind that a sum such as a dollar, that might seem small as of 2011, would have had a much greater purchasing power a century ago.
Errors and Flaws
Despite the best efforts of the U.S. Postal Service, errors sometimes creep into the printing process, with a few of these faulty stamps then find their way to the outside world. Errors range from obvious ones such as the use of the wrong inks or paper and inverted watermarks to near-microscopic flecks. A recent example to look out for is a faulty 37 cent commemorative stamp for the Athens Olympic Games, 2004; its bottom edge is marked with a printer's code. The error is worth in the region of £19 as opposed to 40 cents for the standard stamp.
While used and marked vintage postage stamps survive in surprisingly large numbers, early U.S. stamps in mint condition can be very rare indeed. To assess the condition of stamp, you need to check both sides. Inspect the face for any damage or tears, especially to the perforations, and look for any signs that the stamp has been cancelled by a postmaster, either with a printed frank or by hand. Also check the rear of the stamp for glue stains or other residue -- collectors prefer stamps to be clean and bright on both sides.