Is forgiveness over the phone sufficient to absolve one's liability regarding the rights of others, and is a personal meeting necessary to confirm this, and what should I do with an extra cable whose original store of purchase I do not know?

We ask Allah to accept your repentance and make you steadfast. If your repentance has fulfilled its conditions, then it is accepted. If the person to whom you sold the defective phone forgives you, then there is nothing upon you. Regarding the cable whose owner is unclear to you, inform the owners of the two shops. If they absolve you, then there is nothing upon you. Otherwise, if its owner becomes clear, then give it to him. If the owner does not become clear, then ask them both for the rightful owner to swear an oath to take it. If they differ, then its owner is determined by drawing lots, then he swears an oath and takes it. As for thoughts, whatever of them is within the bounds of Sharia is مطلوب (desirable/required) without exaggeration. And whatever contains exaggeration and severity is from the waswas (whispers/obsessive thoughts) which should be ignored.