EBay buyer not received parcel

Nothing. I had a parcel disappear despite royal mail showing delivered. Someone must have taken it. eBay sided with the seller because of the delivery photo.

Provide the tracking info, wait for case to be escalated, win the case.
if buyer than appeals, make sure you speak over phone to eBay. eBay count as attempted delivery as delivered. however depending in who buyer speaks to, they can overturn first decision. so you need to phone eBay and they will also refund you so both be happy

Is the buyer's address visible on street view? If so does it match the photo of the doorstep?

Obviously if the address had a red door in a brick wall and the photo is showing a green door in a wooden porch then the buyer is probably telling the truth.

As above. U have confirmation of delivery which is all ebay expect of you.

Buyer may have had their item stolen from their doorstep. Does the delivery service you used allow compensation if they sign the form stating item not received?

I had a similar situation 3 weeks ago, scammer buyer thought they could pull a fast one because the postcode showed different on the tracking. I simply messaged them saying it had been delivered and the postcode shown is the DPD depot.
Buyer didn't respond so after 3 days i escalated the dispute and Ebay closed in my favour.

OP: as long as you didn't deliver to an alternative address you are 100% covered by seller protection. If the buyer wants to argue tell them to take it up with Ebay. Don't worry about feedback as it will be auto removed. (edited)

Observations:
1) the hilarious ebay SELLER protection does not require recipient signature, just proof of delivery.
2) Buyer has STATUTORY protection of passing of risk (where STATUTORY trumps any ebay hilarity)
3) EBAY already has proof of delivery from the tracking info YOU ensured was (auto?) provided to ebay PRIOR to estimated delivery date
4) ...sent a largish parcel through Royal Mail 24 tracked(through ebay labelling)

buyer says their was someone in all day and nobody knocked asking for a signature.
He As now opened a request.
What should I do?

Maybe state the obvious: a signature was/is not required.

The square barcode that the postie scans should show up exactly where the parcel was scanned. If the person could be bothered, he could go and find out where it was delivered if he's not pulling a fast one although Royal Mail should get it right first time.

I thought the request would automatically get denied by eBay due to it showing as being delivered, just out of interest is this not the case and you still have to reply etc ?

Yes ,they’re asking me to respond.

Get Proof of Delivery from royal mail and picture. If door is open - it was delivered. If door is shut, then lost by Royal Mail. Tracked means - handed over, not dropped on the street for anyone to take. Good Luck getting money from Royal Mail. Give them 14 days to settle and then
Go through small claims court online. It is faster. In my instance Royal Mail is regularly stealing valuable parcels. I'm not using them or if I do, then Signed For. And stopped using eBay seller account. Those crocks are helping themselves in your bank account. Internet is full of such stories. They refused to hand me over few hundred pounds unless I agree to sign for their banking services. I said no (edited)

Just reply stating item was delivered as per Royal Mail tracking and leave it at that. eBay will eventually close in your favour.