I have had this happen once before with a registered parcel.  I just sent a message through ebay saying I had received their parcel back and asked if they would like a refund or if they would like it resent. I did mention to resend it would cost them the same postage again. I didn't get a response straight away so just held onto it and the money. About a week later the buyer contacted me.

It turns out the buyer who lived alone had been taken to hospital so couldn't collect their mail. I resent it the next day free of charge.