There are few factors that govern how valuable a stamp is to stamp collectors: the most important is how rare it is. Obviously, very old stamps are uncommon so these, such as the 'Penny Black' very seldom come up for sale and so are unbelievably expensive. Other things that might make a stamp rare might be where a mistake was made in the printing and the stamp was withdrawn after only a few had been sold. Undamaged stamps are more valuable than similar ones that have been damaged.
Fact file
In many countries, sets of stamps are issued to commemorate events, such as the 200th anniversary of a person's birth. In the UK, only members of the royal family or entire groups, such as sports teams, may appear on stamps.
