Here's a snippet from the PayPal history:
"In just eight months time, between January and August 2000, PayPal surged from 12,000 accounts to 2.7 million. The company's transaction process helped tremendously to fuel the growth. Money could be transferred to anyone who had an email address, even those who did not have to have a PayPal account. However, recipients did have to open a PayPal account in order to claim their money. The system's convenience and cost won over eBay shoppers. They no longer needed a credit card to buy online, and the service cost them nothing. It was cheaper even than a postal money order and stamp. Sellers were required to pay 1.9 percent of the sales price."