"I have relisted the item but don't know what to do with the auction that had just finished."

The best action may be to cancel the new auction, even if there are bids on it you can still  cancel it if less than 12 hours to the end.  Open a NPB case against first bidder ASAP (ie 4 days after it closed), then in 4 days close the case and relist.  Opening the case may even prompt the orginal bidder to pay.