The Penny Black turns up in inherited collections more often than people expect, usually in a glassine envelope marked “RARE” in a previous owner’s handwriting. It is the first adhesive postage stamp any country issued, it is the most recognizable stamp in the world, and it is almost never the valuable item in the box.…

Rarity in stamps is narrower and more technical than it looks. Five factors account for nearly all of it, and the instincts most owners bring to the question track none of them.

One is the reference standard the whole hobby uses. The other is a free retail price list from a dealer. Both are useful, and confusing the two is how people end up believing a collection is worth ten times what anyone will pay.

A stamp collection has three different values and only one of them is what a buyer will pay. Here is what separates catalog value from retail value from wholesale value, and how to estimate which band a collection falls into.