Unfortunately this is a regular problem, and it's very unlikely to be fraud. 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. Doorstep theft of parcels (known as "porch pirates" in the US) is soaring.
Start by checking with neighbours, and search for any "safe place" where it may have been hidden. If you still can't trace the item you can open a case under eBay's money back guarantee, through the resolution centre. However, eBay can't investigate disputed deliveries. Cases are decided according to the delivery status in the tracking record. If this states that it has been delivered to your address the seller has done all that eBay requires, and they are not be required to refund you. Check the tracking record for yourself and make sure that it definitely refers to your address. If it isn't, you can appeal.
Otherwise all you can do is ask the seller to ask the seller to press Royal Mail or the courier to investigate the loss. Good luck, but if it was tracked as delivered it's unlikely they will accept any responsibility. This has become one of the biggest risks of buying on eBay.
You can read eBay's full money back guarantee policy here: http://pages.ebay.co.uk/help/policies/money-back-guarantee.html
