Amazon ( AMZN), whose shares have slumped 41% this year amid concerns about rising costs and competition, today said it would buy back as much as $500 million worth of its stock over the next two years. Amazon, the No. 1 Internet retailer, has about 418.9 million shares outstanding, according to data on Yahoo! Finance. Wall Street, which has questioned some recent Amazon moves including entering the low-margin grocery business, greeted the news favorably, sending the company's shares up 36 cents to $28.39 early Monday.