With returns, there is no need for any messages to be passed back and forth.
Have you opened a return case? If not, do so now.
Right of the item in Purchase History from more Actions, select Return this item and select the reason for the return.
As you have posted the item back, enter the tracking number of the returned item into the case details.
Once the seller receives the item, they have 48 hours to refund you, through those case details.
If they don't, you need to escalate the case, ask eBay to step in, and they'll force the refund.
Do not close the case early for any excuse or promise the seller may give, and do not forget to escalate.
@alistair7857