1. I came up with the company’s main viral growth mechanism, as David Sacks (PayPal Chief Operating Officer until the sale to eBay) plainly stated to the LA Times (ref. 1). There were other growth mechanisms, such as Max’s automatic buying system on eBay, where he wrote an auction crawler that offered to buy things on eBay only if you accepted PayPal, but my viral email/incentive mechanism was the primary driver of growth.
2. The business model of charging fees only to heavy sellers and not to buyers or light sellers, while still being able to make a profit, was developed by myself and other X.com executives. See details below.