There is no problem in selling the car or any other commodity after you have bought it and kept it in your possession. You can sell it for an instant payment or for a higher deferred payment, whether by installments or not. Allah (Exalted be He) says:
...whereas Allâh has permitted trading
(Part No. 13; Page No. 158) and:
O you who believe! When you contract a debt for a fixed period, write it down.
This includes deferred payment.
Selling a commodity for someone before buying or even possessing it is not permissible, for Prophetic Hadiths in this respect, including one related on the authority of
Zayd ibn Thabit (may Allah be pleased with him) that he said,
“The Messenger of Allah (peace be upon him) forbade selling the goods at the place where they are bought until traders took them to their dwellings.”
The Prophet (peace be upon him) also said,
"Anyone who buys foodstuff should not sell it until they have totally taken possession of it."
He (peace be upon him) further said,
"Do not sell what you do not possess."
Ibn `Umar (may Allah be pleased with them both) said,
"We used to buy food in a lump, so the Messenger of Allah (peace be upon him) would send us someone to forbid us from selling it until we had transferred it to our dwellings."
(Part No. 13; Page No. 159)
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.