[stock-throwaway]

Investors don't have transfer restrictions on preferred shares!!

Only employees have transfer restrictions because they have common shares.

Investors can sell on secondary markets whenever they want, and for the company in question -- they are!

[BaseballPhysics]

And for smaller preferred equity holders (e.g. founders, angels, etc) yeah, they have that option.

I may be wrong, but to my knowledge any significant investor isn't getting liquidity through those private markets. That's all done through traditional bespoke financial transactions. In fact, that's literally what an acquisition is.