Is it permissible to broker a car purchased from an insurance company, without possessing a brokerage license, knowing that the client has bought a tax privilege to reduce customs duties, given that the state prohibits the sale of this privilege and its owner might refuse to transfer ownership of the car?

Your relative asked you to find someone to buy his car, and you are not obliged to investigate its source or how he acquired it. The default assumption for something in a Muslim's possession is that it is his property if he claims so. You have no connection to the risks he might face as a result of purchasing the aforementioned franchise right. If the matter becomes suspicious to you due to doubt, and you wish to abstain from dealing with him, there is no blame upon you, as the Prophet, peace and blessings be upon him, said: "Leave what makes you doubt for what does not make you doubt." It is permissible to buy damaged and defective cars from insurance companies according to the conditions, and brokering them is permissible if the conditions for its validity are met.