Interesting. So Walmart must be making more money with the display ads it serves on its home page through the last year's deal with Yahoo than it would've made by promoting its own merchandise. The entire site served over 900M pageviews in August (Compete Pro data).
I looked at the list of the top 10 retailers, and it seems Walmart's is the only site on that list to serve third-party ads on its home page. It was a pretty big deal when Home Depot was the first large retailer to announce it would start selling ads on its site three years ago. Walmart also serves text ads on its search results pages, via Google.
And so does Target:
AdSense Ads on a Brand Site