What makes a postage stamp valuable

Treasured for their flaws "... Much of coin, currency and stamp collecting, in fact, is based on glorious goofs — everything from inverted images to double and triple strikes in coins to misalignment, fold or crease errors with stamps, Holland says. Take the 1918 24-cent U.S. Airmail stamp with an inverted center showing an early biplane upside-down, for instance. The "Inverted Jenny" is one of the best-known examples of an "invert." ..." [Reference: www.capecodonline.com]