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The business behind Cyber Monday: Inside Fanatics

Knit hats, "ugly sweaters," NHL jerseys, women's apparel and Derek Jeter merchandise.

Those best-selling items are leading the pack at Fanatics.com, the online sports retailer that expects to process 10 times normal volume — roughly 700,000 packages — on Cyber Monday.

In addition to running Fanatics.com, the retailer also powers 300 websites for all the major U.S. sports leagues, as well as collegiate and other professional teams, and some retailers such as J.C. Penney.

"Today will be the single biggest sales day in the history of our company," Fanatics CEO Doug Mack said.

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Wall Street is carefully watching online sales data to get a better read on consumer spending this holiday, particularly after a report from the National Retail Federation estimated that total Black Friday sales over the four-day shopping period fell 11 percent.

Online continues to be a bright spot for retailers, with IBM saying digital sales grew 17 percent over the weekend.

Workers pull Fanatics' customer orders for shipping.
Source: Sabrina Korber
Workers pull Fanatics' customer orders for shipping.

To keep up with these traffic swells, especially on key days such as Cyber Monday, Fanatics is one of many online retailers investing time and money to ensure its website is prepared. This issue was brought to the forefront over the weekend, when Best Buy suffered a series of outages.

Keeping the site free of hiccups is especially important to Fanatics, which draws on the fourth quarter for more than than 50 percent of its annual sales.

"We do synthetic load tests to simulate what this time of year would look like," Mack said. Still, he cautioned that real consumers behave differently than during simulations.

Read MoreNo way Black Friday sales fell 11 percent: Analyst

Backed by big names such as Alibaba, Andreessen Horowitz and Insight Venture Partners, Fanatics.com has been rumored to be on the verge of going public and is valued at more than $3 billion. But for now, Mack said, he's only focused on the fan.

"We have so much growth and expansion opportunity, whether it's across mobile, new innovative forms of merchandise, growing in the soccer arena, using big data," Mack said. "In the short term we're focusing on business enhancements and innovation, and the public question will take care of itself."