It depends on the shareholder agreement, but many do. However, many of these secondary markets facilitate private market transactions via a forward contract which is sort of like long dated put option.
Ah, interesting. I think this might answer a question I posted elsewhere in the thread.
forward contracts and long dated push options differ in their obligations. Put options gives the buyer the right exercise, but in forward contracts they both are by a specific date.
I assume they would only trigger shareholder approve prior to them being exercised not collateralized in the form of the option.
This is generally the case for common stock but not preferred shares (such as you'd get from being an angel), though it obviously depends company to company.
easily circumventable with trusts and other contracts