FACTOR #1 TYPE OF STAMPS

Different types of stamps fetch different prices.

Definitive Stamps - These stamps are the regular small stamps that we are used to seeing all our lives. They are priced at common rates since they're subject to daily, routine use. These definitive stamps can be seen glued at the back of personal greeting cards sent by your parents during Easter or on newsletters that sell none of the things you're interested in.

Commemorative Stamps - If definitive stamps are produced by the billions, commemorative stamps tend to have a more "conservative" production rate and are distributed only by the millions. As you have no doubt deduced by now, these stamps were created in commemoration of a historical event or figure. Because of their limited edition status, they are priced slightly higher than definitive stamps. Commemorative stamps are RARELY reproduced so be sure to buy one of each release when you have the chance!

Special Stamps - This category was simply coined in recent years to describe the stamps that are especially produced for an occasion like Christmas or Valentine's Day. These stamps are bigger than commemorative stamps but their prices - depending on their production rate - can be cheaper or more expensive than the latter.

FACTOR # 2 RARITY - Naturally, the rarer a particular stamp, the higher value it is given. Stamps are classified as rare when there are only limited original copies available in the market. This may be because the stamp has been released long in the past and only a few of it are left existing OR it could be a new stamp with a limited release.
Naturally, one cannot immediately expect to be paid a great amount of money the moment he says his stamp is of rare quality. Experts must be consulted so that the stamp's authenticity may truly be verified.

FACTOR #3 HISTORICAL SIGNIFICANCE - Stamps that have historical significance also command huge prices in the market. The first stamp ever made was the Penny Black in Great Britain and this obviously makes it a collectible item. The first basketball stamp is yet another example; you would have to visit Philippines, however, if you wish to obtain it from its country of origin.

FACTOR #4 CONDITION

Unused Condition - Stamps that are considered unused will generally fetch a better price than one that's used but of the same edition. Secondhand rare stamps however may be tagged at a higher price than unused brand new stamps.
Mint Condition - It's very rare to see stamps that are truly in mint condition, especially those that are more than a decade old. A stamp in mint condition is one whose original beauty has been preserved throughout the years. No cuts, tears, folds or any other flaw mar its appearance.
Stamps in mint condition will always sell at a good price because of the amount of care bestowed upon them by their owners.

FACTOR #5 AUTHENTICITY

There are a number of organizations and experts today that can provide generally accepted certifications of authenticity for stamps. These documents serve as proof of the stamp's validity and directly contribute to the added value of the stamp.
If you wish to ensure that auction houses will peg your stamps at the price they deserve, make sure you procure all the necessary papers to completely prove your collection's genuineness!