Definitely bookmarking this, many thanks! I wonder how it compares to https://www.bookfinder.com.

Also, any chance of adding shipping information (e.g. the prices shown would also include shipping cost to a predefined location)?

Annas's Archive also has a very comprehensive metadata search engine for books,

The person who manages to create full-text search for the books in the shadow libraries will have created the most important tool of the decade. If there is any effort being made in this direction, please let me know.