One possible reason for Amazon's slower sales growth is that it's starting to lose its price advantage relative to competitors. A recent study by pricing firm 360pi and Wells Fargo compared 100 items at Amazon to nine traditional retailers, including Wal-Mart and Target. Although Amazon is still widely the price leader, Wal-Mart and Target had lower prices in the clothing and shoes, electronics, housewares and health and beauty categories.  Read MoreAmazon doesn't always offer the best deals: Study  At Wal-Mart, for example, prices have gone from an average 1 percent lower than Amazon's, to 10 percent lower in the most recent quarter. Macy's prices were only 1 percent higher than Amazon's in apparel and shoes, compared with 17 percent a year ago.  Earlier this year, Amazon boosted the price of its Prime membership to $99 from $79. Although this likely wouldn't dissuade the company's core users from the service, it might discourage people who were considering signing up, said Ken Perkins, founder of Retail Metrics.  "I really am baffled by ... anybody who thought that Amazon's chances for unlimited earnings potential was so large," Mulpuru said.  That's not to say that Amazon's woes are in no way tied to the broader economy. RW Baird analyst Colin Sebastian said in an email that he doubts Amazon is losing share to bricks-and-mortar competitors that have stepped up their online game, as it isn't the first time there's been talk about competitors improving their capabilities.  Instead, in a note to investors, he listed sluggish device sales and certain macroeconomic headwinds as two reasons behind its revenue guidance of 7 percent to 18 percent growth in the fourth quarter.
Since last Christmas, retailers have taken aim at Amazon by improving their online shopping capabilities and chipping away at its price advantage.