I've worked with two shopping search engines, and interestingly, scraping sites was one of the things they did to build up their inventory as well. The big difference being, they simply organized the products into a searchable format, then sent traffic to the ecommerce site and let them handle the checkout . What you're doing is arguably more complex.

(They also prioritized the feeds that were sent to them directly by retailers above the scraped items feeds - thus prioritizing paid listings, similar to the Google SERPs - so a different business model entirely.)

That being said, a very cool concept - and agreed that, given the relatively-small number of ecommerce platforms out there, scraping then erving them up seems pretty scalable. Interested to see how it goes.