They could have disclaimed it if they didn't want this problem. It's an interesting issue though, because if they were able to sell it at some point, it'd be worth millions and millions. Is it fair to let them inherit it tax free when it has such a high value that simply can't be realized right now? Could these people get a waiver to sell it?
If they win their case against the IRS and subsequently sell the piece, then they would presumably be doing so with a basis of $0, and would therefore be taxed on any income they received. The rub here, is that the amount of tax assessed in this situation would be significantly less than what is owed under the estate tax.
What's interesting, is that the IRS
seems to be ignoring its own regulations when assessing this piece at such a high value. IRS regs state that:
The fair market value is the price at which the property would change hands between a willing buyer and a willing seller, neither being under any compulsion to buy or to sell and both having reasonable knowledge of relevant facts. The fair market value of a particular item of property includible in the decedent's gross estate is not to be determined by a forced sale price. Nor is the fair market value of an item of property to be determined by the sale price of the item in a market other than that in which such item is most commonly sold to the public, taking into account the location of the item wherever appropriate.
26 C.F.R. § 20.2031–1(b)
I would say that a very relevant fact is that it is against the law to sell the piece, which is why Christie's valued it at zero. Now, we know that the decedent obtained a waiver to possess the piece, but can the heirs obtain a similar waiver to allow them to sell it? If they cannot obtain this waiver, then I think the IRS's assessment of its value is on shakey ground. If, however, they could obtain the waiver but simply do not want to part with the artwork, then I think the IRS stands a strong chance of prevailing on the issue.