Costco has been a great retail stock, but is it a buy?

Costco Wholesale ( COST -0.78% ) is not only one of America's favorite retailers. It's also been a top retail stock to own over its history.

Over the last decade, it's up 570%, easily outpacing the S&P 500 's increase of 240%, and most investors would argue that Costco is lower-risk than the broad-based index. After all, the retailer is a classic defensive stock. As a consumer staples company , it sells primarily products that people need, like groceries, paper products, and health and beauty products. It's also known for its buy-in-bulk bargain prices, which attract consumers in both good times and bad.

Costco has actually underperformed the market this year as it's up just 4% through Sept. 19, pulling back from its rece

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