Bellow you can see the list of books prices search engines I've found.
FetchBook.Info new & used books price comparison - Compare books prices among more than 60 books stores. Includes shipping. Easy to use.
ISBN.nu - Compare prices at several online book stores. Both new and used books.
Biblio.com - Finds prices for new and used books.
PriceFarmer - A guide to new and used books prices among 4 book stores.
PriceLook.co.uk- Compare books prices, as well as other products.
Some other good books resources
AldoBooks - Some good resourses on used books. Several cheap used books store.
Nova Book Resources - A couple of good used books resources could be found here too.
How to sell used books
Book Reviews- Many book reviews.
Biology Books- Resource for used biology books.
Used Books Resources- Several used books resources.
Pets Books - Everything you need to know about your pets.
Computer Books- Find any tech book you might have looked for.
Cooking Books- Several good cooking books, not used though.
Student Books Online- Student online bookstore.
Faculty of Cinema- Cinema related bookstore.
Faculty of Psychology- Psychology related bookstore.
Faculty of Computer Science- Computer Science related bookstore.