This looks really interesting, though if one is looking for publicly available data, I think it's an open question of how much of the value is in providing a marketplace vs. actually doing the aggregation, organization and some analysis.

Two other interesting startups in this space that are focussed more on the data aggregation end are http://www.AnythingResearch.com and http://www.AggData.com (which has dozens of retail datasets just like the Walmart store example, presumably gathered by screen-scraping).