I'm very price sensitive. I have plenty of books I haven't read yet, so it is very rare that I need a book promptly.
So I use the camels. http://camelcamelcamel.com/
is a website that keeps track of the many items sold by the various Amazon websites. Typically, you can see what the price has been over the past year or more for Amazon, its Marketplace Sellers, and its Marketplace "used" Sellers.
I typically inform the camels that I would be interested in a book when its price reaches the point of 25 cents higher than its lowest price of the past year. When that happens, the camels emails me, and I then often buy the book.
I believe that the camels receives money from Amazon when orders are placed when going through the camels' click-through button.
Book prices at Amazon fluctuate frequently, and I typically save $2-4 dollars per book.