In general, there isn't that much stock on the market (millions/billions of dollars worth) that an outside investor can get his hands on. So, if Buffet had bought dexter using cash, the investors couldn't have just turned around and used that cash to purchase Berkshire stock.
Exactly - if you look at the numbers, you'll see that BRK.A is a very low-volume stock (today's volume was 1,700, which is quite high for that stock). It's also got only 1.5M shares outstanding, in comparison to (for example) IBM's 1.3B.
