Great idea for a thread Ogg!
This is my prize posession:
It might not look like anything much unless you collect the Discworld stamps, but it's a very rare LBE (Little Brown Envelope) that contains one of each of the first series of stamps, including the rather rare Blue Triangle.
There were only about a dozen of these particular LBEs made and sent to Harper Collins in the US as prizes for the best display of Going Postal in book stores. Sharlene used to own a bookstore and she got one. There are only around three known to still exist.
We were talking about the stamps several years ago and Sharlene mentioned she had some and did I think they were worth anything? In those days I thought it was just a passing gimmick.
About two or three years ago one of these LBEs was sold on eBay for $400. Lord knows how much one would go for now.