Mark Cuban vividly remembers the moment he realized he was nearly broke.

He was 27 years old, trying his hand at entrepreneurship for the first time with a software company called MicroSolutions, when he plugged his PIN into an ATM and got a rude awakening, he said on a recent episode of TikToker Bobbi Althoff's "The Really Good Podcast."

His company only had $2,000 left in the bank. The culprit: His secretary, who had been stealing from him by using white-out and a typewriter to forge company checks, he said. The secretary took about $82,000, effectively wiping out MicroSolutions' account balance, Cuban confirmed to CNBC Make It.

That meant Cuban's own money was nearly gone, too. He bootstrapped MicroSolutions almost entirely, only accepting external funding in the form of a $500 advance from a former customer, the New York Times reported in 2017.

"It was f---ed up, but [it was] the best thing that ever happened to us because it made us get our s--- together," Cuban told Barstool Sports' "Pardon My Take" podcast in 2020, adding that all he could do was try to remain calm and get back to work: "You have to hustle the most when you think it's the darkest."

It didn't take long for Cuban to recover the cash, and then some. His day-to-day work wasn't always glamorous, he wrote in a 2004 blog post: He spent hours delving into software and Cisco router manuals, and "sitting in my house testing and comparing new technologies."

The hustle — coupled with the fact that he was selling software around the time home computers became popular — paid off when Cuban sold MicroSolutions to CompuServe for $6 million in 1990. He was 32 years old.

Cuban then helped co-found AudioNet, which became Broadcast.com and was acquired by Yahoo for $5.7 billion in 1999, making Cuban a billionaire at age 40. He currently has an estimated net worth of $5.1 billion, according to Forbes.

Even if Cuban lost that fortune overnight, he has "no doubt" he could become a "multimillionaire again," he told NPR's "How I Build This" podcast in 2016. That's because his ability to sell made him rich in the first place, he added.

"I would get a job as a bartender at night, and a sales job during the day, and I would start working," Cuban said.

Excelling at sales can play a "significant" role in building financial success, historian and sociologist Rainer Zitelmann wrote in his 2018 book, "The Wealth Elite: A Groundbreaking Study of the Psychology of the Super Rich."

About 15% of CEOs from the top 100 Fortune 500 companies started their careers in sales, according to a 2017 Heidrick and Struggles survey. It's the third-most common background among U.S. chief executives, tied with operations and behind finance and engineering.

"Once you learn how to sell, you can always start a business, [because] you're an entrepreneur at heart," Cuban told The School of Hard Knocks last year.

Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank," which features Mark Cuban as a panelist.

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