The price of a non-marketable good

Came across this crazy piece of news. A pair inherits an art piece ("Canyon") worth $65 million and has to pay an inheritance tax worth $29 million. The problem, however, is that the art piece includes a stuffed bald eagle, which cannot be sold since the bird is protected. (Obviously, if a stuffed bald eagle could be sold this would only encourage people to hunt them.) In other words, "Canyon" cannot be sold due to the stuffed bald eagle and yet the pair is stuck with a tax bill of $29 million. So how can it have a price tag of $65 million? How can a non-marketable good have a market value (the price of collectibles is obviously based on their market value, not some out-of-the-hat valuation)?

Even more worrying, the IRS is acting like the vengeful, petty bureaucrats that they are:

Placing a value on an item that cannot be sold is no easy feat. The venerable auction house Christie’s placed the value of "Canyon" at zero. The IRS initially put it at $15 million, then jumped the figure to $65 million when Sundell and Homem refused to pay, according to The New York Times.
The IRS, which declined to comment on the matter, is not only asking for $29 million in taxes, but also an $11.7 million “gross valuation misstatement” penalty, according to Forbes.

Inheritance taxes are nuts anyway. What exactly has the federal government done to deserve $29 million out of "Canyon"? They're simply robbing someone's estate.