Have a meta-site, like a pluggin, with a shopping cart. You browse other sites such as Amazon, EBay, Anandtech, etc and it creates a registry of things you want.

On the backend it can solicit better offers from other merchants on the same and similar merchandise.

Also, as you note, the customer's data could be sold and (partially) turned into a rebate. Perhaps the site could also tell you which extra data (your gender, age, other purchases, etc) would be the most valuable and you could choose to release these facts.