When you don't receive an item you can open a case under eBay's money back guarantee, through the resolution centre: https://resolutioncentre.ebay.co.uk/
The problem is that anyone can sign for parcels, it doesn't have to be you. Some services don't even require a signature. Parcels may be left with neighbours, delivered to the wrong address or left in a "safe place" to be lost or stolen. "Porch piracy", or the theft of parcels from the doorstep is soaring.
This is a depressingly regular situation. You need to do everything you can locally to try to locate the parcel, e.g. check with neighbours and have good look around outside the property.
EBay can't and doesn't attempt to investigate disputed deliveries. They decide cases according to the tracking record, in accordance with their money back guarantee policy. This requires the seller to provide a valid tracking number that records successful delivery to your address. If it states that it was successfully delivered, the seller has done all that eBay requires and they are not be required to refund you.
EBay's money back guarantee policy forms part of the user agreement. You can find the full details here: http://pages.ebay.co.uk/help/policies/money-back-guarantee.html
