If there's no estate tax, there's no reason the government should have to know what the estate contains, outside of registered transferrables such as stocks, bonds, and titled property.
Thus, if a painting is handed down, and the heir sells it for a mega buck or two, there's no reason the government has to be bothered about it. The government didn't do anything to create the art, the government didn't have an interest in the art before, the government didn't take any risk in the ownership of the art, and the government didn't have any emotional attachment to it. Thus, if an heir sells an inheritance as a private party, the government gets nothing, because it's none of the government's business in the first place.
The fact that a valuable property exists is not sufficient reason for the government to steal part of it.