There is a new book buying service available- Booksprice.com enables one to search a book across many sellers. I liked the feature that enables tracking using RSS feeds. According to an email that I received:
Booksprice.com is a free service of finding the best price on books among the major online stores. Our real time engine can help you save as much as 80% from the retail price on new & used books,cds and dvds. We have introduce innovative features and tools like finding the best price of several books together, which might reduce the high shipping costs and save even more money. And now we are offering the new “RSS Price Watcher” that enables you to track the price of a specific book and with your own preferences (shipping, currency, condition, etc) using RSS feeds.
Results of a search on Don Quixote in Edith Grossman’s translation published about three years ago. The price shown is inclusive of shipping charges in the US.