Hi and welcome to the forum

In your last posting, the overprinted orange stamp is an 'official' stamp & has a CV of $1.05 in the 2009 Scotts Catalogue (#O28), the 25 Pfennigs blue (#476) has a CV of $1.00 in the same catalogue. The Hindenburg is (either #413 or #430) has a CV of $1.00 apiece. There are two different watermarks with this issue.
At one time, it was not uncommon for every collector to have large amounts of Germany in their collections. They were easily obtainable for a lot of the issues.
Germany is still one of the countries where a lot of their stamps are still easy to buy, with exceptions of course, with the scarce & rare issues. A lot of countries can still be bought in volume for the cheaper values.
A lot of duplication is caused buy buying different collections and each time you do, there are always duplicates and they don't take long to acccumlate.
Chimo
Bujutsu