This is why i always send the buyer the tracking number the day i ship the item. there is no need for you to refund the money if the item was lost by the shipping company, but if you have no proof of shipment that is a different story.
I actually keep a database of all sold items, what i sold, who i sold it to, what they paid, what i paid for shipping, when i shipped it, and the tracking number. This way if anyone comes back at me i have all the info i need.