I recall this topic was brought up before, somewhere in a thread. Apparently some countries have set an 'expiry date' for the use of stamps denominated in their pre-Euro currency. IIRC Finland is one of those countries, so collectors and dealers are scrambling to 'use up' their stocks and such material can be had in the "scrap postage" market for a huge discount on face, as once the clock runs out, all you can do with them is add them to your collection, and in the case of modern material, pretty much everyone that wants one, has one.
I wouldn't say that covers using pre-Euro denominated stamps in the Euro period will be worth anything. Why would they? As long as the stamps are valid for postage, they are not postal history displaying illegal use of stamps. They're simply philatelic mail, and would be worth less than covers showing the stamps used in-period, as a stamp used years later cries out "I'm a cover mailed by a stamp collector who bought their postage below face from a dealer".
Yes there was a topic somewhere.
I think a few countries are banning the use of pre-Euro stamps at the end of 2011 (I believe Finland is one such country) as the public have had 10 years to use up all those old stamps.
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