Is the seller justified in refusing to refund part of the price to the buyer who discovered a new defect in the car after the sale and an inspection that negated ignorance (of the defect)?

If the buyer discovers a defect in the car after the first inspection, and the defect was present before taking possession of the car, then it is the seller's responsibility. The seller has two options: either to return the full price to the buyer, or to pay him the value of the reduction caused by the defect. However, if the defect occurred after taking possession and was unrelated to anything prior to possession, then there is no guarantee on the seller.