There's two problems I think that would limit the value of this as a service. The first is that any consumer product offered somewhere with a "lowest price gua

There's two problems I think that would limit the value of this as a service. The first is that any consumer product offered somewhere with a "lowest price guarantee" or "price match guarantee" is likely to be a unique product for the retailer. Stuff like low cost appliances and electronics are notorious for this. A corollary is that many product brands have exclusive distribution deals with retailers, which is stuff like lightbulbs, yard equipment and power tools, or really anything you can buy at a hardware store (compare stuff at Home Depot, Menards, and Ace for example). The second is that the other trick retailers pull is by selling "exclusive" products that are really just the same product with a different name. Furniture is a great example of this, there is even a startup out there (the name escapes me) that catalogs the same furniture available from multiple retailers online by different names. Reverse image searches of the product can get you pretty far here. Just a random example: I needed kitchen appliances. I have a big incentive to lower my cost here, but also specific features I want out of each item. Refrigerators are the worst about this, if I wanted a particular combination of product series, finish, ice maker, door config (French door, freezer in left or right, etc), and form factor it would essentially determine which store I would buy it from.