The founder of Nasty Gal explains why she didn't buy anything when she became a millionaire

Before Sophia Amoruso founded Nasty Gal - which started with her selling vintage clothes on eBay and grew into a $100 million-plus clothing business - she spent her teens dumpster diving and shoplifting.

In fact, the first thing she sold online wasn't clothing - it was a stolen book.

"Books were an easy entry point for a novice shoplifter like me," she writes in her book "#GIRLBOSS."

Today, she has no reason to shoplift or dumpster dive: She's worth an estimated $250 million.

The self-made millionaire started to see major financial gains in 2010: "After Nasty Gal moved off eBay and transitioned to a full-fledged business, I had almost $1 million cash in the bank," she writes.

While seven figures would cue most people to go on a shopping spree, Amoruso kept her wallet closed.

A "big no-no is increasing your spending as soon as your income increases," she explains. "I have always been careful to avoid this pitfall."

Part of her resistance to lifestyle inflation came from the fact that she simply didn't have time to spend. "For a long time I was so focused on growing the business that spending money on myself didn't even cross my mind," she writes. "Even if I had wanted to drop $500 on a pair of shoes, I was just too busy."

She essentially tricked herself into saving money, she explains: "When your time spent making money is significantly greater than your time spent spending money, you will be amazed at how much you can save without even really thinking about it."

And besides, she says, "You can't act like you've arrived when you're only just receiving the invitation."