Where are you getting this? The plaintiff does not need to establish value. The public corporation needs to be very transparent in disposal of assets for fair market value.
Any shareholder can demand an accounting of how the public company established their price. If the company refuses to adequately demonstrate that they obtained value for the disposition, a court could compel them to do so. Imagine the scams a public corp could pull off if this were not so.
If the public corp did not act in the shareholder's interest, they would become liable for damages. It would be easy enough to show that they got full value - if in fact they did.
BTW, I am an unlicensed attorney - meaning I have a juris doctorate but don't practice law. You?