[recursivecaveat]

It's usually not that different from just amount * marginal market value, such that its not generally worth worrying about the distinction. The main reasons you can't dump 1 trillion dollars of stock at once is it would be difficult to find enough buyer capital quickly, and announcing your intention to do that is going to drop the price as you're implying an overvaluation.

Both of those problems can be solved if you just do it slowly in a pre-announced manner. A trillion is a thousand billions, so you could cash out 1 billion dollars annually until the 30th century assuming zero growth/inflation. So unless you're really worried about the optics of selling 0.1% of your holdings, or plan to spend more than 1 billion a year outside your business, you're not really constrained.

Everything is liquid in a long enough time horizon. It just depends on what you mean by liquidating. Like do I have to sell my house today? In a week? Months? More flexibility = more value.

For Musk the situation is definitely more extreme though. Since a significant part of that valuation is attached to him owning it, he would definitely take an abnormal level of haircut. Probably an unknowable investor sentiment question though.