Collectors of used postage stamps are called philatelists. They seek out both used and unused stamps of historical or other significance. While collectors value some stamps at only a few cents, others have sold for hundreds of thousands of dollars.
When collecting used stamps, philatelists look for errors in the stamp printing process, such as incorrect dyes, off-center printing and text errors. Used stamps that contain a historically significant postage mark may carry additional value. The value of a used stamp varies depending on its condition, with stamps removed from envelopes without tearing or thinning worth more than those missing most of their gum from their back.