[nostrademons]

The point is that lots of other people are selling shares too - by the numbers, several times more than Bill and Ballmer. Yet nobody cares much if say, Fidelity or Vanguard or Goldman Sachs is selling.

In a liquid market, somebody is always selling. And somebody else is buying. In order to have a transaction, you have to have both a buyer and a seller. As long as those balance out, the price remains stable. You only see wild swings in price when there are suddenly many more sellers than buyers, or several more buyers than sellers. That doesn't happen with these planned insider transactions, because as soon as a sale is announced, a dozen computerized arbitragers swoop in to buy up the shares they've just sold (as long as the market doesn't consider the insider sales to be a vote of no confidence in the stock).