By selling these shares, RS is hedging his bets that his exchange will not be around that long. If he trusted in his exchange, the wise move would be to keep all the shares.
That would defeat the purpose of having shares in the first place...
If RS thought his exchange would be around for many years to come, he would not be selling shares. He would be keeping them. That tells you how confident he is in his own exchange.
This reminds me of when ASICMiner sold their 10 first blades, and people went insane and started bidding over $8000 a unit. Why would they sell something that was basically printing them money? Because they knew that avalon chips were about to go on the market, and they had already developed their nextgen ASICs. It is unlikely that those who foolishly purchased one of their blades recooped even 20% of their initial investment.
In other words, the present value of these shares is far more than the shares themselves will ever generate. You are all getting fucked over. Mark my words.