Has the seller given you a tracking number that has shown delivery of your item to your address? Have you checked and confirmed this?
Then if it was a Royal Mail delivery they have been known not to leave a ' calling card' so it's worth checking your local sorting office ( not Post Office) if your item is awaiting collection there.
Have you tried to open a case for item not received from the Resolution Centre? If the seller does not have that tracking showing delivery, you are entitled and will receive a full refund here.
