In reading the honorable and esteemed David Weinberger's latest Journal of the Hyperlinked Organization, I encountered a snippet about Jakob Nielsen's latest book, E-Commerce User Experience (order via Amazon.com). Mr. W. cites a few of the rules that Nielsen enumerates, two of which are:
11. Don't show products that customers can't buy.
51. Show total cost, including taxes, shipping and handling, as soon as possible.
Without articulating it, I have done both of those things (if not others) at isbn.nu in the refresh I gave to the programming in September 2001. The old site displayed results even when the book was unavailable that a user was trying to find prices on, and couldn't show details like sales tax and shipping. The new version only displays entries from stores that claim they can ship the book, and users can select their shipping state (in the US), country, and even exchange rate to get the full price for a single book.
Nielsen trafficks in hard-won truths, many of which are counter-intuitive but are based on direct observation of individuals. Most of us work anecdotally not empirically, and thus substitute personal preference for research. (Look at Steve Jobs and the color white.)