X.Com und Paypal (1999/2000)

Musk reinvested the money from the Zip2 sale in his start-up X.Com in 1999. The goal was to build an online bank.  To do this, the company developed an online payment system that allowed people to pay by e-mail.  

In 2000, X-Com and its start-up competitor Confinity, which had developed Paypal, merged. From then on, the companies operated only under the Paypal name. 

eBay acquired the payment service in 2002 for $1.5 billion - $180 million of which went to Musk. He reinvested that money, too: he put $100 million into SpaceX, $70 into Tesla, and $30 into Solar City. Read more about the companies in the further text.