I have Greek stamps overlaid with the eagle & swastika; I was told they were worth less because they WEREN’T canceled.
I got sold off binders of coins years ago to put together a down payment on a house; they were recent issue, very common. I held onto pre-1945 stuff, as well as more recent ones if the countries no longer existed (Rhodesia, Upper Volta, etc.). Great way to learn geography/history as a kid.
I have dozens of uncanceled Nazi stamps, but I don’t think they are rare; one in particular probably is (Hitler & Mussolini facing each other from the sides, with the German phrase “One people, one struggle” on it.
You’ll have to search the internet re the value of German occupation stamps today but when I was collecting seriously, (started in 1952-60s), they were regarded as historically valuable and probably in short supply re destruction and/or lost during WW2 period.
There are collectors of German/Nazi memorabilia so they might be a source of potential customers. These stamps are also valuable as teaching items, showing today’s kids what life/history was like during the 1930’s/WW2 40’s and “why” they were marked as occupation stamps. There is a whole history lesson behind them re Danzig, Czechoslovakia, etc.
They like to see things up close and even to touch them esp. if they have a historical context.
If I remember correctly, even normal German stamps were reused after WW2 during the Allied occupation, marked by occupying power/zone (American, British and Soviet).
Some regular stamps, such as the British, often had a black band around them to signify a royal death, marking a historical event. I believe other countries also had this practice.
For very early stamps, the postmark, very often handwritten, can make a certain stamp valuable. The same for perforation holes.
Other stamps such as the Persian airmails of the 1950’s or early 60’s (with the Shah’s head on them) were exquisitely colorful, while the Vatican and/or Spanish “Catacombs” large-sized stamps were printing works of art. Ecuador had a masterful stamp series of their president and I think, George Washington, Lincoln and/or FDR on them. Mexico had something similar.
The skill of lithography printing has been lost on this computer-generated reproductions generation who don’t appreciate the work that went into printing not only stamps but book plates and even documents.
I am constantly finding fascinating documents at the National Archives, even from the early-mid 1900’s, where the printing and the writing were absolutely beautiful. For the educated, they are items to be admired. Stamps, in their own way, are worth looking at as more than just a piece of paper.