The item you sold to the buyer who claimed not to have recieved it. Was it sent as a letter or parcel ? If it went as a parcel that was mailed over the PO counter it would include a tracking number. This should be on your Post office reciept. You can log this number into the Aus. Post website to see if the item has been delivered to your customer. You can also enter this tracking number into ebays system using the drop down menu next to sold items.