If the reality is as you have mentioned, that you conclude an agreement with the person who wants to buy the car on credit by fixing its value and that he gives you its description then you fetch it for him later, then this transaction is prohibited. This is so because
you are selling what you do not possess. It is authentically reported that the Messenger of Allah (peace be upon him) said:
A loan and a sale are not lawful (together), or two conditions in one sale, or to make a gain of something that is not yet secured, or to sell what you do not have.
(Part No. 13; Page No. 246)
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.