I recognize that Michel does a great job listing varieties, especially for European philately. And I do see a lot of the European sellers pricing material based on a percentage of Michel, which is MUCH higher than Scott pricing for stamps listed by both sources. Some American sellers have gotten into the act- they list material by Scott then quote a catalog value which comes from Michel to make you think they are selling at a reasonable "percentage" of catalog.
All that said, material priced by Michel (say in the range of 40% of cat and up) sits for a really long time unsold. I have been watching some Netherlands sets on
ebay for years that have never sold or had their price modified. The seller involved but be wealthy beyond any reckoning, as he is not generating cash flow or inventory turns from his "rarer" material.
So what is rare, and what is rare material worth? It takes time to acquire this knowledge for a given area. My deepest knowledge for pricing is in the KGVI era for British Commonwealth. In this area, Bermuda keyplates showed intensive pricing growth from the late 1980's until perhaps the mid-2000's. Then, several very large holdings of Bermuda keys hit the market making most printings of the keyplates much, much easier to find. Still, sellers held onto established pricing "habits" based on the past, and only very reluctantly began to lower pricing. Even today, keyplates are generally overpriced as SG has not adjusted prices to reflect the market. And why should they? They are a retail stamp business that needs to protect their investment. In the background though, if you are selling they explain the market forces behind much lower buying prices, but they surely do not tell you this if you are buying from them.
As I am learning about new areas of philately, I have realized (with A LOT of help here on SCF) that other countries are cataloged way beyond their market price. Some examples are Germany, Austria, most if not all of Scandanavia, Benelux, and Liechtenstein among others. This may be due to reduced demand in the US which seems to have a withering collector base for European stamps.
The rule of thumb I am developing and learning is to see if certain material is available from US sellers. If it is, then it is overpriced in Michel. If you can't find it in the US (something I see with Spanish and Portuguese Colonies, and certain French Colonies), then the Michel value is somewhat more in tune with the marketplace.
Just some thoughts.