How to judge the condition of a stamp?

To evaluate the condition of a stamp, first look at the front. Are the colors bright or faded? Is the stamp clean, dirty or stained? Is it torn or creased? Torn stamps are not considered "collectible," but they can be used as space fillers until you get better ones. Are the perforations intact? Has the stamp been canceled? A stamp with a light cancellation is in better condition than one with heavy marks across it. Is the stamp design centered, crooked, or off to one side? Centering can range from "superb" (perfectly centered on the stamp) to "good" (the design on at least one side is marred somewhat by the perfs). Anything less would be graded "fair" or "poor" and, like torn copies, should be saved only as space fillers. Centering varies widely on older stamps. An examination of the back of the stamp will reveal whether it has been carelessly treated and thus is less valuable.

You can read more about stamps here: Where to Buy Stamps