The Seattle-based online retailer has come under attack from Wall Street bears, citing concerns over operating margins, but Amazon may have put some of those concerns to rest in its most recent quarter. Amazon's fourth-quarter operating margin was 1.9%, compared to 1.5% in the prior year's quarter.Strong revenue growth and the continued growth in market and mind share have allowed Amazon to trade at more than 70 times 2013 earnings estimates, far higher than its brick-and-mortar retail rivals such as Wal-Mart (WMT) and Target (TGT). Amazon offers a variety of products that its competitors can't match, like free two-day shipping with Amazon Prime, and a burgeoning ecosystem with its Kindle Fire tablet and e-readers. Amazon Prime is a real hook for consumers, who spend $79 a year to get goods faster than traditional shipping. Amazon Prime also offers content, such as the first two seasons of "Game of Thrones," at no extra charge, making the service more valuable. The stock is up 6.2% this year. Shares closed on Thursday up 1.2% to $266.49.