Given all the problems at BJ's Wholesale Club (NYSE:BJ) and the lackluster results at Wal-Mart (NYSE:WMT), it wouldn't surprise me to see shares of Stock Advisor selection Costco (NASDAQ:COST) getting held down a bit, even though the business has continued to outshine its competitors.

But that's not at all the case. Shares of Costco continue to be reasonably valued by my estimates, and sales results, such as those reported by the company in December, continue to be strong as well.

For the all-important month of December, Costco's overall same-store sales increased 9%, with a 10% increase at international locations and a 9% increase domestically. Compared to last year, there was one extra day of sales in the period, and the company estimates same-store sales results benefited from this by 3%. But even a 6% result is substantially better than the 3.5% result at Wal-Mart's Sam's Club warehouse stores.

In some respects, Costco's results aren't a surprise. Costco's customers have higher income levels compared to most retailers, and other high-end retailers, such as Nordstrom (NYSE:JWN) (which also saw same-store sales increase 9%), had strong holiday results as well.

Going a level deeper on Costco's results reveals that sales were consistent throughout the month. Business was strongest in the Northeast, Northwest, Midwest, and Texas in the United States and in Korea, Taiwan, and the U.K. internationally. Currency gains benefited results by 1.1%, compared to 0.75% last month, and the average transaction was higher by 4%, while traffic increased by a little more than 4.5%. Anytime a retailer can move both the transaction and the traffic needles, it's a great thing to see.

As a shareholder of Costco, I'm always curious to see how the company is doing, and in the past month I visited two stores myself. I visited one store in Arlington, Va., and made a good attempt to visit the one in Chiba, Japan. The store in Arlington was very busy, which is normal. According to Kimco (NYSE:KIM), which owns the location and leases it to Costco, it is among Costco's top 10 stores in sales. The store in Japan that I attempted to visit on a Sunday was so busy that after waiting in line for 30 minutes to enter the parking garage, I gave up. Having been to that particular store before, I can attest that it is at least as busy as the Arlington store on weekends.

A channel check of two stores isn't ultimately very meaningful as a predictor, but taken into account with the overall results reported by the company, it helps show that Costco's offerings continue to resonate with shoppers. Given this, its expansion potential, and its reasonable valuation, I'm inclined to be a shareholder for quite some time.

For more on the warehouse-retailing leaders, check out:

At the time of publication, Nathan Parmelee owned shares in Costco and had a beneficial interest in shares of Wal-Mart, a Motley Fool Inside Value selection. He had no financial interest in any of the other companies mentioned. The Motley Fool has an ironclad disclosure policy.