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What is the fair value of a stock given the bid and ask prices? Is there such a relationship?
If you need to show that the sale/purchase was at FMV, then showing that you made a trade on a public exchange with an unrelated counterpart is enough to establish FMV. However, this is only one of the possible "fair market value" definitions. This is usually used to determine basis or value for tax purposes. For valuation purposes or general accounting, one specific trade is not enough to establish FMV, and much more research is required.