Much of Bill Gates worth is held in Microsoft shares and I assume he has many of them.
I doubt if he is allowed to sell them (at least not all of them) as this would send a bad message to the markets. He can probably only sell so many a year.
If Microsoft went bust tomorrow and his shares became worth nothing his personal wealth would drop alarmingly.
some of the stuff he owns is quite expensive, bet he owns a couple of satelites and the odd jet, maybe a few streets and possibly an island or 2, it all ads up i guess.