Is it sinful to demand the return of money in cases where the product—which arrives later—does not arrive, or arrives with a defect, and how can the matter be rectified?

If a buyer finds a defect in an item after purchase, he has the option to either return it and reclaim the price, or keep it and take compensation for the defect. Your demand for a refund, if based on truthfulness, does not prohibit you from taking the money awarded by the platform. As for the product that reached you after its price was refunded and you were unable to contact the seller, this amount is a debt upon you that must be paid to its owner. If you despair of reaching him, then spend it on any public welfare or give it as charity on his behalf.