Buyer return item missing

Hi Ebay,

 

I am new to eBay selling. I sold an item on eBay and after 2 weeks the buyer decided to return the item. He posted back the item and requested a refund on eBay. It says on the tracking number it was delivered but I did not receive the item and eBay said I have 5 days to refund the item. 

 

I contacted aupost they said they are investigating the case.

 

Should I refund the buyer or wait for the item? 

If the item is lost in transit how can I have my item back? or the cost of item

 

Thank you.