I had a friend build an eBay app so you could scan things while at Walmart to see if you found cheaper products, but that's as far as he was able to get with his app, he couldn't do any pricing comparison against Amazon etc because they would revoke his API access...

It's really a shame, because it would 100% be nice to have a one-stop shop that you can find the exact same SKU product and figure out who has it the cheapest.