In the area of stamp forgeries, one has to be very careful when collecting postmarks. Fake cancels abound, especially on Italian States stamps, German States stamps, early Swiss stamps, German Colonies stamps, and Western European stamps of the 1920's, where the used condition prices are much higher than the mint condition prices.
To benefit my visitors, I would like to introduce you to an excellent site that covers all aspects of forgeries, philatelic expertization, and faked cancels. It is a Finnish website, however most of the pages are in English.
This website contains pages on all of the known philatelic experts, their authentication markings, FORGED authentication markings, famous forgers and forgeries, prominent faked cancellations, and on the famous Blum Forgery case.
Here is a quick link to the site page on fake cancels. I have this bookmarked and use it frequently. There have been several times, where I've almost purchased a German Colonies stamp with a rare cancel that I've though was a terrific bargain. Then, after checking this page, I have found that the cancel was suspect. Doing this has saved me a lot of disappointment AND money, in the long run.
This link features the some 650 forged postmarks made during the early 2000's that were revealed during the Blum Forgery Case.
There are also other postmark links for Germany, Finland, and the British Commonwealth. There is an interesting exhibit section, as well, which shows examples of many philatelic forgeries.
are still great rewards out there, in collecting postmarks. One just
has to educate themselves and use care when buying high-priced
Another way to buy rare postmarked stamps is to make sure the particular item either has a certificate of authenticity or that the item is "signed" or "back-stamped" by a well known philatelic expert for that particular "area". Many European collectors are very particular about this type of material and will absolutely not buy anything that isn't authenticated. Many European sellers only sell authenticated material of this type. It can be a bit more expensive, but at least the buyer can be confident that what he is getting will be authentic.