The main thing that makes a stamp valuable, is its rarity. If only a very few of them were issued or are known to exist, or if some mistake was made and a few of these reached the public, the stamps are likely to be quite valuable. One famous rarity is a one-penny orange Mauritius stamp of 1847. Instead of having the word ‘Postpaid‘, it was inscribed ‘Post Office‘. But most collectors can hardly hope to obtain such rare and valuable stamps.
A stamp catalogue today lists about two hundred thousand different stamps. Damaged stamps are not worth very much. Unused stamps without gum are not in demand and can usually be bought cheaply.