Use data from purchases with electronic payments to generate data about sellers with the best prices. This could report to you where have you or others bought stuff and cheaper, and use those benchmarks to search for equal or better offers. For stuff you don't need immediately, you could put things on a wishlist and the desired price, and get notifications when that criteria becomes available. An example to a degree could be the website findbookprices.com, which allows you to create wishlists, set prices, and reports last best recorded prices for books. I am sure there are other good ideas that people can add to something like this, or maybe even suggest existing resources.