Is permissible to buy/sell with non-Muslim?
Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.
Should the Zakah (obligatory charity) giver tell the poor recipient that this is the Zakah of his money?
No, he shouldn`t. But, the Zakah giver should make an intention in his heart that it is the Zakah of his money when paying it to the poor. And Allah Knows Best.
What is the ruling of Sharia on marrying a Jewish woman, dealing with the Jews, and going to Israel?
All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is allowed for a Muslim to marry a Jewish or a Christian woman provided that this doesn`t affect his religion. As for dealing with the Jews who are outside Palestine, it is the same as dealing with the Christians; it is permissible as long as it doesn`t harm Islam. As for dealing with the Jews who are inside Palestine, the ruling differs depending on the persons themselves and the situations. By the end of the day, a believer`s heart leads him/her to what is good. And Allah The Almighty Knows Best.
Is it lawful or unlawful to take payment of key money?
Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If the lease contract has expired and the lessor refrained from renewing it, then the tenant is obliged to leave the property for nothing. And Allah The Almighty Knows Best.