What should I do with the money I owe to a person from whom I bought an item, but I can no longer find any trace of him after my travel and his travel, and he has not contacted me despite my leaving him my address?

You must search for the owner of the money and pay off his debt to him, based on the Prophet's (peace be upon him) saying: "Every Muslim's blood, wealth, and honor are sacred to another Muslim." If you are unable to find him, then give the money in charity on his behalf. If you find him later, give him the choice between taking the money or considering the reward of the charity for him.