Last week, while covering comparable-store sales at Wal-Mart (NYSE:WMT), I mentioned the results from Motley Fool Stock Advisor selection Costco (NASDAQ:COST) in passing. But since I own shares, I decided to circle back and take a closer look.

The basic numbers show a same-store sales (comps) increase of 6% for the entire company for the five-week June period, with U.S. comps up 4% and international comps up 14%. In local currencies, the international comps were up 11%, with Korea and the U.K. leading the way.

But the more interesting portions of the sales numbers are further into the company's release. Transactions were 7% higher in dollar terms, but the volume of transactions was flat to down 1%. The trend in transaction volume, or traffic, is similar to what Wal-Mart has been reporting the last few months: Consumers are making fewer trips, but making it up for it by spending just as much as they would otherwise. Costco is often cited for having higher-income customers, so it's not surprising that the effect would be muted at Costco, but it's a definite point of interest amid continued high energy costs. It could also just be seasonal or an aberration, but it's worth watching in relation to other retailers and how shoppers behave.

Costco sold gasoline at an average of $2.87 a gallon in June, versus $2.13 a year ago; this year's higher gas prices benefited same-store sales by 1.9%. In addition, the company also noted that comparable-store sales were off by about 1%, because the bulk of Fourth of July sales will fall into the July period this year, and that cannibalization from opening additional stores in core markets impacted same-store sales by approximately 1.8%. Cannibalization is something to watch for, but this is something the company actively monitors to maximize profitability and efficiency for its markets.

Overall, I see nothing worrisome in Costco's sales results. Even if I did, I'd find it extremely difficult to justify a buy or sell decision on one comparable-store sales number. For that reason, I much prefer companies' 10-Qs and 10-Ks. Judging by those numbers, I still prefer Costco as an investment over BJ's Wholesale Club (NYSE:BJ), but for a mix of wholesale clubs and discount retail, I don't think Wal-Mart should be ignored at today's prices.

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