There’s more than merchandise for sale this holiday. The shoppers are, too. Walmart, Amazon.com, and other major retailers that keep troves of data on shoppers’ purchases are selling more digital ads on their own websites and in stores — including pop-up banners and search-bar keywords — to marketers eager to pitch their products. The retailers want to tap their insights on consumer behavior to attract ad dollars from packaged goods manufacturers like Procter & Gamble and Kraft Heinz at a time when companies face new challenges in targeting marketing to people online. Read more at Reuters.