Contact the seller and apologize. Ask that he send you a Mutual Cancellation for the unwanted items.
He doesn't have to agree, but the MC ends the transaction immediately that you agree, which is faster than it would take to complete an Unpaid Item Dispute (around a week) and freeing up his stock faster. He will also get his selling fees back.
If he does agree leave positive feedback for the cancelled transactions "Thank you for dealing with this promptly and courteously" makes you and him look good to future customers.
