The same could be said about real estate. A certain sized house in a particular neighborhood may be going for $500K, but in reality only a small number of houses are on the market at any one time. There is no way everybody in the neighborhood can sell their houses for that price all at the same time.
As global currencies weaken the likelihood of liquidating your position for "fair market value" deteriorates.
As long as you have a fair amount of liquidity in the markets it could be argued that the price you get will be "fair market value". Today fair market value is approximately $790. Tomorrow it may be $1000 or $10. If you are getting approximately the same price as all the other guys who are buying or selling at the same time, then you are getting fair market value.