Today, Mark Cuban's business acumen has made him a fortune: He's worth an estimated $3.7 billion, according to Forbes. He started honing those skills early, as a kid growing up in a working class family in Pittsburgh.
"People thought I might go work at a mill. My mom wanted me to learn how to lay carpet because she was concerned about my future," Cuban said on an interview for ABC's "Shark Tank," on which he is an investor.
"Nobody had high hopes for me," Cuban said. "But I was a hustler."
Cuban sold baseball cards, trash bags and anything else he could get his hands on. One side hustle that left a particular impact was collecting and selling stamps. According to the United States Postal Service, which interviewed the billionaire about stamps, Cuban has even said stamps helped him pay for college.
"I first became interested in stamps when I was about 12 years old," Cuban told the USPS. "My mom had an album of old foreign stamps that she started collecting when she was young – I still have that album today."
Since stamps were inexpensive, it didn't take much money for a kid to get started researching stamps, learning which ones were valuable and selling them.
"I went to the post office. I went to stamp stores. I bought packages of stamps from vendors I found in the back of magazines — wherever I could find them. I spent hours sorting through the stamps," Cuban explained. "I liked to find the rarest stamps that I could afford."
It turned out to be an experience that would have lasting benefits.
"Collecting stamps is an amazing way to start to understand business. Each stamp has its own level of scarcity, of demand, of price, and as a collector you have to make decisions on when to keep a stamp, trade or sell it, and when to invest in a new stamp for your collection," Cuban explained. "I learned so much about business and the laws of supply and demand when I was still in middle school that business came easily to me when I got to college and beyond.
"I bought, sold and traded so many that the experience taught me as much about business as any class I have ever taken," he added.
Today, Cuban's advice is to succeed is to start learning something and get to work at it. "Never stop learning. Never stop grinding. Never stop loving every single minute of your life," he told CNBC Make It in 2017.
In January, he even offered up an easy side hustle for young people today to try.
"[In my honest opinion] the easiest money right now for anyone 14 or older is digging in to be an expert in Amazon Alexa and Google Home, and every compatible device," Cuban tweeted. "Then, I would offer to consult and configure the apartment and homes of everyone I know, and everyone they know."
The most important lesson ambitious young people can learn is the determination to hustle, Cuban says: "You've got to grind. It takes work. There are no shortcuts."
Like this story? Subscribe to CNBC Make It on YouTube!