Senh: SecondMarket is an original concept. The idea is so simple, which is probably why it's catching on. The only problem I see so far is that it really over-inflates valuations of private companies like Facebook and Twitter. Their valuations are 50-100 times their revenue. It allows employees to cash in on the hype of their companies, rather than on actual revenue.
Senh: A lot of people will be losing money. It's way overvalued. That's 75 times annual revenue.