If the seller's item hasn't been returned yet:

Contact the buyer to find out where the package is.
The buyer is responsible to ensure that the package is returned to the seller in the same condition the item was received in.
The buyer can view & Print invoices just the same as the seller can making the return simple for buyers.
The buyer must send returns with track and trace, this provides evidence that the items have been sent out for return back to the seller.

What happens if the buyer used the item, or it's damaged while it's being returned:

It's the buyer's responsibility to make sure that the item is returned in the same condition and ensure that it's packed properly and protected against damage during delivery.
Sellers may deduct an amount from the buyer's refund to cover the loss in the item's value.

What happens if the buyer doesn't send the item back:

If the buyer doesn't send the item back then you do not need to send any refund back to the buyer

I decided to keep the item:

If a buyer has changed their mind about requesting a return, the buyer should send the seller a message to let them know.