Complex sentence: Postage stamps are also object of a particular form of collecting, and in some cases, when demand greatly exceeds supply, their commercial value on this specific market may become enormously greater than face value, even after use.
Split into simpler sentences: Postage stamps are also object of a particular form of collecting. In some cases, when demand greatly exceeds supply, their commercial value on this specific market may become enormously greater than face value, even after use.