So you sold something.
The buyer was not satisfied.
You told the buyer to return it for a full refund.
The buyer did not return it.
At this point, you don't owe him any money. Don't worry. He has only 45 days to get the item in the mail and he has to use Confirmed Delivery service. You are not required to refund his return postage.
OR
You refunded the money and asked him to return the item.
He hasn't.
He never will.
This is what is called a 'learning experience'.
You will never get the item back. Because you refunded, eBay assumes that both of you were satisfied that the buyer's claim was valid.
You will not get the refund back.
Not every transaction will go perfectly.
While is is a good idea to agree to take returns and to make refunds, because your selling account can suffer badly if you do not, it is important to get the order of actions right.
First GET THE ITEM RETURNED
Then and only then REFUND THE CUSTOMER'S FULL ORIGINAL PAYMENT.
Keep in mind that some sellers are comfortable with refunding on reciept of a picture of the unwanted item, for example broken china. So it is not unusual for eBay to see refunds without returns. But you are within your rights to demand the return and the return is normally at the buyer's expense.