Is there an API that can find books from multiple sellers by say, ISBN? It would be nice to easily find the cheapest book from various sellers.
It compares prices amongst Amazon: different country sites price things differently, the site above compares them with postage. It's often cheaper to buy from a different country.
http://www.getinvisiblehand.com/
When you visit a product page on Amazon or other retailers, a notification bar will show you the lowest price for that product.