Calculating the tax basis value based on what the owner would sell it for has numerous problems even outside that subjectivity. It penalizes businesses for success in a way that promotes looting the company to artificially lower the business value. Aside from the deadweight loss involved, it artificially promotes the type of behavior firms like Bain are already criticized for. Also, it's highly subject to system-gaming. What is the incentive of a firm to actually accurately value it at what they would sell for while it was open? The only way it works is to enforce some draconian policy that requires them to sell if someone ponies the cash (a la Posner and Weyl's proposition in Radical Markets).