The agreed price for the last executed sale is the de facto value of anything traded. This has been a fact for hundreds of thousands of years by now.
In your specific example of other side - yes - just because someone who needs to sell a chunk for reasons like an emergency, retirement or consumption needs doesn't mean they are happy to sell the rest of the chunks at that price.
Market based valuations only work in case of very high liquidity publicly traded assets and only if you don't own a significant %.
This makes your argument weaker, not stronger though. If there isn't liquidity market based valuation doesn't work.
>>This has been a fact for hundreds of thousands of years by now.
It isn't and never was. Liquidity was always big part of it.
If you by some hacker magic know of any way to circumvent this fundamental logic, then you will become the richest man in history within less than a year, since you will then buy for less than anybody is willing to sell for and sell for more than anybody is willing to buy for.