If they are denying receipt, and R.M say it has been, you just need to check that RM delivered it to the correct place, so ask in what name it was signed for. It has been known for them to ask a neighbour to sign if you were out, in which case you need to ask the neighbour. It has also been known for them to just deliver to the wrong address. Again, checking the name signed should determine this.
If the name matches, then they are obviously trying to pull a fast one, so just restate royal mail has the signature, and therefore it has been proven to e delivered. If they take it up with ebay, then you will need to provide ebay with the tracking detais and proof from RM of signature, and you should be fine.