Am I obligated to rectify the damage inflicted upon the buyer due to selling him a defective device without informing him of its defect, and what should I do if I am unable to locate the buyer?

The seller must disclose any defects in the merchandise before the sale. Whoever conceals a defect sins, but the sale remains valid. Repentance for this act is obligatory, and the buyer must be informed of the defect. The buyer then has the option to either annul the sale or keep the item. Their option is forfeited if they become aware of the defect and accept it. If the buyer cannot be reached, then seeking forgiveness (from Allah) and giving charity equivalent to the value of the defect on their behalf becomes obligatory.