A market price is the monetary value offered for an item or service and accepted.
An unaccepted offer is not a market price since it is not accepted.
Fair point. Google tells me the bid/ask on Amazon stock is $232/$235. So if some hypothetical Asset Assessor Service tries to use market conditions to assess the worth of Bezos's 900 million shares, they'll figure out would-be buyers would rather have Bezos's stake than $209 billion, whereas would-be sellers would rather have $212 billion than Bezos's stake. So the AAS would presumably assess Bezos's stock for tax purposes at somewhere between those numbers. Which is to say, an offer to buy that is not accepted
is a lower bound on what an asset will be assessed to be worth. And both the upper bound and the lower bound are measures of how much other people want Bezos's stuff.