There must be many descendants of Survivors living in the USA who locate property that belonged to their parents in Poland, and then seek to inherit it and then sell it.
That is what I did. Well, the sale is pending.
I will consider the central question here first. For the further complications, I would be willing to pay you for your services.
Central question. My understanding is that capital gains are due on the proceeds. Cost basis is value on day of inheritance. I went to Poland in 2006 to appear before the Polish court and the court recognized me as an inheritor sometime after. But as there are several family members each with different dates of recognition, then when did I inherit it? And how dow we know what it was valued at that time? As the Real Estate market was much higher then, could it be that the realized profits will be negative? How to know what it's value actually was on the day of the inheritance? So the questions are: 1) What if the date is not clear, and how to arrive at a cost basis properly?
The complication is simply that ownership never actually passed to grandmother and mother as they were already deceased before the rules of property inheritance became possible in Poland- as my GM dies in 1984.
Thank You.

Normally you inherit the property on the date of death.
For value at the time of inheritance, use your own judgment based on facts.
If you have to pay taxes in Poland, you will file form 1116 to claim foreign tax credit. Your U.S. Tax Return: Tax on Inheritances (http://taxipay.blogspot.com/2008/02/tax-on-inheritances.html)