Yahoo! is really rolling out API / XML tools to help the developer community build useful Web resources for their Web visitors. Yesterday, they released the Yahoo! Shopping Web Services which enables one to build "applications that provide comparison shopping and product search functionality." They give you three ways to get at the data (1) search by product, (2) search by category and (3) search by merchant.
Forget simply about comparison search...Think about using pricing data, in real time, to be competitive. Competitor A decreases price of product Y by $1 under yours. You can then dynamically change your pricing to $1 under them. Obviously, one would need to make sure there is a minimum profit margin left, but that is an easy algorithm to build.