There are a few places where you can post want lists more or less permanently. They can be scanned by booksellers, other collectors, or anyone passing by. It's not really the most efficient method, but you never know, and it doesn't cost anything.
Meta Search Tools
Books & Book Collectors
Advanced Book Exchange
Zentrales Verzeichnis Antiquarischer B�cher
You can search directly from the following search forms. Just enter either author or title in the form and click on the search button.