I actually don't even get this. Surely this problem is solved with Liquid Stock or a voluntary buyout (if allowed) at the next funding round? Most institutional investors don't want anyone cashing out until a liquidity event like a buyout or an IPO. I would imagine the revenues available for this private transfer market are very small.
The revenues are very small for the company but could be huge for the employee.
If you joined [random company] early at got 1000 shares that are worth 10x much after a round or two of funding, it may have turned into house downpayment-sized dollars in the meantime. Further, most employees don't know if their company is raising until it's a done deal. Therefore, they can't predict, plan, or act when the time is right.
Regular, predictable liquidity windows is compelling and gives employees more choices, even if they never take them.
