And remember that the only purpose of a tracking number is to protect the seller against false claims of non-delivery.
If your purchase is coming from Canada or the USA and more than 20 days have passed, use Contact Seller to ask for the date of shipment and the service used.
If the seller is in Asia, allow 30 days.
If the answer is not satisfactory or if you are ignored, go to the Resolution Centre at the bottom of this page and open an Item Not Received Dispute.
The first thing you will be told to do is to contact the seller, but you have already done that, so go on to Step Two.
If the seller cannot prove delivery, you will be refunded.
If more than 30 days have passed since payment, you may be going to Paypal rather than eBay.
But probably the item is just on its way and either the parcel or a Delivery Notice will show up in your mailbox any day now.