Costco Wholesale Corp (COST) - Get Costco Wholesale Corporation Report entered the ecommerce arena in the same year Amazon.com (AMZN) - Get Amazon.com, Inc. Report started selling more than just books in 1998, but Costco's sluggish pace of growth on the web could leave it in the dust of larger sites including Amazon, Walmart (WMT) - Get Walmart Inc. Report , Best Buy (BBY) - Get Best Buy Co., Inc. Report and Target (TGT) - Get Target Corporation Report , Bloomberg reported.

Costco's comp sales for locations opened more than a year soared over 5% at the start of 2017, but ecommerce sales were up a modest 11% compared to Target's 22% growth, Best Buy's 23% growth and Walmart's 63% growth. The overall ecommerce market jumped 15% annually.

Costco sells 6,000 more items online than it does in stores, but brick-and-mortar performs better with Costco's sample-loving, internet-averse older set of bread and butter customers.

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Half of Costco shoppers are Amazon Prime members, up 14% from five years ago, Bloomberg reported. This growth in competition is troublesome enough, but it's made worse by Amazon's pending acquisition of Whole Foods Market (WFM) . Since Costco shoppers primarily frequent the warehouses for groceries, Amazon's increasingly large footprint in the industry is a potential threat.

Costco stock traded up 0.4% premarket to $160. Shares are just down for the year, slipping 0.46% since the start of 2017.

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