What is the expiation for a debt I owe to a seller whom I have not found after promising him to pay the remaining price of the commodity?

This amount is a debt owed by you to the seller. If you despair of finding him or his heirs, then donate it on his behalf. Ibn Taymiyyah said: If a person despairs of knowing the owners of usurped properties, or borrowed items, or deposits, or pledges that were in his possession, then he should donate them on their behalf, or spend them on the welfare of Muslims, or hand them over to a just distributor who will do so.