In Tuesday's Wall Street Journal, I'm mentioned (and quoted) along with a number of other entrepreneurs who run price-comparison engines for book prices. (That's a free 7-day link to the Journal to read that article.) Addall.com is the granddaddy, having launched in 1998. They're pretty good now. They look at 41 bookstores. My site, isbn.nu, looks at 17, but they've got a big tent and I'm pretty picky.
The article gives a good flavor of the pros and cons of finding the best price, and the range of arbitrage that's available. A smart consumer might be able to buy books from Overstock and sell them on eBay or Half as new, but the market usually is too efficient for that to work.
The reporter who wrote this piece focuses on the media and publishing industries, and the article represents the point of view of the publishers rather than the booksellers, for the most part. For instance, Amazon.com used to tolerate, barely, we price comparison engines. The CEO once made a pretty negative remark about them, in fact, before they settled on their current combination of discounts and free shipping. But Amazon.com and other stores now completely encourage us.
I drive well over $1.5 million in sales to the stores I list, although I don't exactly gross or net five percent of that--some stores pay bounties for new customers; others pay on an entire market basket; others pay only on direct clickthroughs. And I have to pay for servers, freelance programming help, co-location, and book data. At the end of the day it's completely worthwhile.