What is the ruling on taking a commission from the buyer upon completing the sale of cars belonging to the company I work for, knowing that this is not among my primary job responsibilities at the company?

As long as the car is not in your possession and bringing the buyer is not your job, you are not an agent for the company in this sale. There is no harm in you agreeing with the buyer to take a commission from him upon completion of the sale. The condition for the permissibility of taking the commission is that only the payer knows about it; the other party, who will not pay anything, is not required to know.