Perhaps the best place to start is:
http://philadb.com/?site=list&catuid=484Most inflation issues are listed and prices given.
Unfortunately, there is a complication to selling German stamps, and that is that most Germans will not buy valuable stamps that do not have certificates of authenticity. The prices at the above site, for example, are stated as applying only for stamps with INLFA certificates.
I recently had a bad experience that illustrates the pitfalls of trying to sell better German stamps. A couple of weeks ago, I listed three Germania issues with CVs of over 100 Euros on
ebay: two sold for very low sums (one bidder only each) and one did not sell at all.
In Germany certificates are expected and a certain amount of suspicion attaches to stamps demanding high prices which have not been expertised.
Here is an example of a current
ebay auction that's doing well:
http://cgi.ebay.de/Berlin-UPU-Serie...em19c39b8c8cNote that these stamps have been expertised by Schlegel.
If you want to get the prices your items deserve you will almost certainly have to fork out to get them expertised first.