Does the seller sin if he does not mention in his car sale advertisement that it was previously involved in an accident or stolen, and is he obligated to respond to the buyer's demands to return the car or pay him compensation after the sale is completed and the buyer becomes aware of the repairs?

Disputes are to be settled in court, not through fatwas. It is not permissible for the seller to conceal significant defects in the item sold. The reference for determining a significant defect is the people of experience and specialization. If you did not conceal significant defects, then there is no sin upon you. If people of experience testify that the defects are significant, then you must respond to the buyer's request to return the car or compensate him for it. When the item is returned due to a defect, the entire price is refunded to the buyer, unless the buyer accepts the defect after becoming aware of it. Your acceptance of its return is considered an annulment [of the sale], for which you may take compensation.