Amazon.com’s stock dropped in Wednesday’s premarket trading 1o.6% to $90.16, after some analysts said they were concerned about the online retailer’s lower gross margin in the third quarter.After Tuesday’s regular market close, Amazon.com reported results that topped Wall Street expectations. But the company also said shipping deals and discounts for copies of “Harry Potter and the Deathly Hallows” caused gross margin to slip to 23.4 percent in the third quarter from 24.3 percent in the second quarter.

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