With Amazon.com, Inc. (AMZN) completing its $13.7 billion purchase of Whole Foods Markets (WFM) , the e-commerce juggernaut is officially entering the bricks-and-mortar grocery space.

With other moves, though -- like its teaming up with Sears Holdings (SHLD) to sell Kenmore products exclusively on Amazon -- it's clearly showing that Amazon is looking to dominate just about every aspect of retail.

However, the stock hasn't been acting that well as of late, falling roughly 7% over the past month and about 10% from its all-time highs of $1,083.31. The reason? Investors may be becoming concerned with Amazon's gross margins as it continues to enter new markets, TheStreet's founder Jim Cramer, who also manages the Action Alerts PLUS charitable trust portfolio, said from the floor of the New York Stock Exchange Monday.

New markets equals lower gross margins as the company looks to take share from its competitors. We saw this on Monday with Whole Foods, as Amazon began discounting some products by more than 40%.

Because of this gross margin pressure, Cramer is leary of AMZN stock. "I myself do not want to be in Amazon," he said, adding, "it's my least favorite FANG stock." He suggested investors read some of the recent work from Doug Kass and Bruce Kamich on Real Money to get a better sense of the retail environment.

Amazon has been a big concern for Best Buy investors, too. The retailer is set to report earnings on Tuesday before then open and one thought is that BBY stock could post a big breakout, possibly rallying into $70 to $80 range.

The "smart money" has already pushed BBY stock to 52-week and all-time highs, so that probably means the quarter will be good, Cramer said. However, the concern is that the talk will shift to Amazon potential hurting the business and that could hurt BBY stock in the process, he reasoned.

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