How does the buyer get 70% of the price? I don't understand. 1. I sell my XBox (~$200 value) on sillytrader for $100. 2. pg puts it up on his site and through

How does the buyer get 70% of the price? I don't understand. 1. I sell my XBox (~$200 value) on sillytrader for $100. 2. pg puts it up on his site and through referrals gets $20 of the $100 sale price. 3. You see it on paulgraham.com and buy it for $100... and this is where I get confused. Where does the 70% come from?