If things start going south in a public investment, a shareholder may not be able to exit that, either. The truth is, it's all about liquidity, and liquidity dries-up on the way down.
Sure they can except in the most extreme corner cases. You an contrive situations in which it's difficult to sell stock on the NYSE or NASDAQ, but it's basically always quick and easy (e.g., you can move $100K of FB stock in seconds using your phone during just about any market hour and often even outside of them). That is definitely not true of private markets, at least at the moment.
Next time the market is in melt-down mode -- like it was just beginning to be last October -- watch how wide the spread gets. Sure, if you're somebody willing to take any price in a fire sale you can get out of anything.
I'd consider that a corner case. And even then, it is possible to get something out. With private investments, that just may not be true or involves high ad hoc transaction costs.
My original reply was intended to point out that what was the top article comment at the time completely missed the point. Cuban is arguing that severe liquidity restraints are bad, especially so for small time investors. For scenarios like you describe, public exchanges aren't perfect either, but they are very, very good at facilitating near-instantaneous liquidity and they strictly dominate the current set of private crowd funding vehicles.
