While Priceline pioneered the "reverse-auction," brokerage model of travel deals, many different industries and sites now work on the opposite standard: By listing deals and prices from around the Web, customers can be assured of getting the lowest price on any specific product or package they desire. Kayak searches the Web for all travel offers -- airfare, hotel, transportation -- based on trip specifications you select, then produces a list of options for the customer to compare. If you don't like a particular air carrier or seating option, for example, you can compare other similar packages to find the one best suited to your preferences and needs.
Likewise, installable browser add-ons like Invisible Hand -- or simple shopping results inside some search engines, such as Google -- can automatically show you the lowest prices for any product the second you begin searching for it. While there are often showcase or featured suppliers who've paid an incentive to come out on top of these lists without regard to whether their prices really are competitive, it's simple enough to compare top results and find out for sure.
While eBay has made offer aggregation and price comparisons a major part of its online auctions and buy-it-now sales, bringing the concept into the mainstream, Amazon has combined regular and offer-aggregate sales into its pricing for every product: Search for a book, and you'll get prices for other online retailers, used booksellers, and even private individuals selling single new or used copies at a stated (non-auction) price.
While incentives exists for consumers to use Amazon's own trailblazing fulfillment setup (including Amazon's shipping discounts and reliable customer service), if you're simply looking for a low-priced, used copy of a book, this internal price aggregation can be a lifesaver. After all, even "Super Saver" shipping isn't always as cost-effective as paying a simple $4.00 for one of millions of books. That's 1 cent for the book, $3.99 shipping -- and it's a price you'll see on tons of books all over Amazon's site.