What should the seller pay the buyer to clear his conscience after selling a car and explaining its defects, then the buyer discovers other defects that were not clear at the time of sale, in addition to the cost of repairing a defect about which the buyer was misled?

If you do not conceal from the buyer the defects you know of, and you were unaware of those that later appeared, then there is no sin upon you, and you are not a deceiver. However, if the defect is significant and reduces the the price, then the buyer has the right to keep the item and receive compensation for the defect, or to cancel the sale and reclaim the price. If the buyer accepts the car with its defects without demanding compensation, then there is no blame upon you.