All perfect 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.
It is permissible to use family assembly`s funds in buying and selling transactions and, according to scholars, this is called (Murabaha to the purchase orderer). However, this is provided that the members of the assembly approve of this and that you take possession of the commodity, become liable for it in case of damage or the like, then sell it in cash or by installments for a certain profit that you agree on with the customer.
As regards Zakah, the assembly`s funds are liable for it once a full lunar year passes over their possession and since the funds are joint money, then they are treated as one unit when it comes to paying their Zakah. And Allah The Almighty Knows Best.
* A Murabaha to the purchase orderer (MPO) is a form of Murabaha in which the potential buyer (purchaser/orderer) instructs the seller (usually an Islamic bank, a financier) to purchase a given asset/commodity according to pre-defined specifications.