Is zakat obligatory on unsaleable goods and overdue debts? And what is the ruling concerning the zakat on these monies if their zakat was paid for previous years, then they were sold or collected?

There is no zakat due on inherited trade goods for the heirs, because zakat on such goods requires their ownership with the intention of trade. So, if one acquires them through inheritance or without a prior intention, and then intends to trade with them, they do not become trade goods (for zakat purposes from the time of inheritance). If, hypothetically, zakat were to be due on them, then for partners, zakat becomes obligatory on each individual's share separately, if their share reaches the nisab (minimum threshold) or if a hawl (full lunar year) passes over it. The depreciation of trade goods does not waive the zakat due on them.