How can I compensate the person who bought my mobile phone card, which was sold unlawfully, knowing that I do not know who he is or where to find him, and two years have passed since then?

You are not permitted to reclaim your phone card after selling it, because if a sale is concluded with its conditions, neither party may void it except with the mutual consent of the other. As for the action you took, the value of the card reverts to the first buyer or to whomever he gifted it, and the last buyer has no right to the card. The balance on the card belongs to the one who deposited it, and you are not allowed to benefit from it. You must search for its owner, and if you do not find him, then give it as charity on his behalf. If he then comes and approves the charity, then all is well; otherwise, you are liable to him for it.