When you return an item it all has to happen through the correct case opened for the return. You do not pass a tracking number to the seller, but you enter it into the case details.
If you haven't done so already, open the relevant case for the return, enter the tracking number and when it shows delivery the seller has 48 hours to refund you.
If they don't, you escalate the case, eBay step in and force the refund.
@jabi_3851