revenuecollector's find in
this thread got me thinking -- do you have any high-value stamps that you got for a bargain price, either because it wasn't properly identified, wasn't recognized for what it was, etc.?
Two from my collection came to mind:
I bought this set of four for about $2. They are the German-occupied Serbia set issued on 22 Sep 1941 to benefit the victims of catastrophic damage to the city of Semendria. Michel catalog price for MNH, 8 (about $11 US).

As it turns out, the 2+4 Din stamp has a rare plate flaw -- the "front door" plate flaw -- not listed in Scott (which the seller was using), but listed in Michel's specialized catalog. I didn't see the plate flaw until months later when I scanned the stamp for posting to my website.

Michel catalog value with the plate flaw, 450 (about $620).
Perhaps my favorite catch... These are the 1941 Banja Luka local issues from the Independent State of Croatia (German puppet state in Croatia during WWII).

Michel catalog value for the pair is 1,500 (750 for each). I paid about 400 for the pair. Once I got them home, I realized the 2 Din stamp had a rare plate flaw -- "B
asanska" instead of "B
osanska".

Michel catalog value for the 2 Din with that plate flaw -- 3,000.
Too bad I'll never see a dime of profit off the finds. Maybe my heirs will.
