In this Nov. 28, 2014 file photo, a Target shopper uses her iPhone to compare prices at Wal-Mart while shopping after midnight in South Portland, Maine. Increasingly, buying products online is like trading stocks: you can buy a copper mug or a coat and then hours - or even minutes later - it can go up and down in price. (AP Photo/Robert F. Bukaty, File) Photo: Robert F. Bukaty, Associated Press / Associated Press contributed 