Here is an idea for someone who wants to make a website:
A site that records (daily) the prices of everything from every store. Users can find out where the cheapest place is to buy a given item, and you can also see the sales history of said item at any given store. You can find the trends of when said item goes on sale and what the lowest sale price has ever been, when it was etc so that you can figure the best time and place to buy something.
As the owner of the site, you partner with the best sellers so that you can send traffic there and get paid for doing so.
Its been (mostly) done but its a little harder because there is a lot of difference regionally. I am sure you can buy that data from a broker but that will come with a yearly cost that would be too much for a startup unless you had some money behind you. (Have worked with data brokers before).