I know this is cliche, but it wouldn't hurt to add social networking capabilities. This is actually an idea of mine I applied to YC with two cycles ago. It was a social network for product reviews and product prices where users would gather in interest groups (gaming pc's, music rigs) and bookmark the products they have, or want, on their profiles. Users would then jump from user to user to see what set up others are using. Each product would have it's own page with a rating, reviews, and scour the internet for the best prices. Not only would you index new products by scouring the net, but users would be motivated to find new products and submit them to your site.

More realistically, however, I think a feature that allows users to bookmark products and receive alerts when new lower prices have been found would increase stickiness. Imagine a user coming to your site, bookmarking a product and sort of forgetting about it, only to find an email alert telling them a lower price has been found, and they're back.