How can a seller avoid sin when not returning a defective item if he suspects that the buyer may be dishonest or was the one who damaged it, knowing that the buyer has the right to return due to a defect even if the phrase "no returns or exchanges" is present?

If a defect is found in the sold item, the buyer has the right to return it. The presumption is that the defect existed before the sale, unless proven otherwise by evidence. In case of disagreement between the seller and the buyer regarding the time the defect occurred, experts should be consulted. If they cannot determine anything, the presumption is that the defect is old, unless there is evidence to the contrary.