How do people know that satoshi hasn't already sold some of his coins ?? 
Maybe because the blockchain is public, and they have already tracked his addresses. How did they track those addresses?
How many coins are on his addresses?
i think it was because he mined the first blocks..
if BTC is $1000 for 1, and he's a billionaire.. then he must have at least 1 million coins. but i believe it's set at 1.5 million. that's a lot of money that can go to good causes. i hope he donates it one day, since he can't use them.