I bought a car that had a simple defect and I sold it without telling the buyer about the defect. Is this a kind of deception?
Yes, this is a kind of deception. It is well-known that deception is prohibited. It was authentically reported that the Prophet (peace be upon him) said,
He who deceives us is not of us (is not my follower).
You have to repent to Allah and seek His forgiveness. You also have to hasten in telling the buyer about this defect in order to acquit yourself. If he does not relinquish his right, you have to agree with him either to pay him the value of this defect or to take the car back and give him the price he paid. But if you do not agree, then the judge in your area can pass a judgment in this regard. But if you can not reach the buyer, you have to pay the value of this defect as charity. (Part No. 13; Page No. 205)
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.