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where do all the stamps come from?
Well, you've got 175 years of philatelic accumulation we're talking about. Stamp collecting has been a pretty popular hobby for a very long time. Given the numbers of people who currently collect stamps plus the all the people who have ever collected stamps (i.e. estates), there are probably several dozen to hundreds of collections (of various size and quality) hitting the market in any given week, just in the US. Most collectors who have collected for a long period of time also tend to accumulate various duplicates and unwanted items or they sell off part of their collection to finance a new one, etc. A truly staggering amount of material has accumulated over the last couple centuries with more being added every day. Even a minuscule percentage of that total still amounts to a huge amount of stamps trading hands every day.