While art lovers may appreciate the I.R.S.’s aesthetic sensibilities, some estate planners, tax lawyers and collectors are alarmed at the agency’s position, arguing that the case could upend the standard practice of valuing assets according to their sale in a normal market. I.R.S. guidelines say that in figuring an item’s fair market value, taxpayers should “include any restrictions, understandings, or covenants limiting the use or disposition of the property.”