I suspect that 'delivered' simply means that the system assumes that it's been delivered because there were no known delays in deliveries to that address and nobody has contacted Royal Mail to say that their package hasn't arrived.
There's no barcode (or similar machine-readable ID) on a package which isn't sent either 'Signed For' or 'Special Delivery', so there's nothing for the postie to scan when he/she delivers it. Therefore it's impossible to know for certain whether it's actually been delivered or not.
So the buyer might be telling the truth when he says that he didn't receive it. (I once ordered a DVD from eBay which never arrived. The seller swore that he'd sent it but still agreed to send out a replacement, using 'Signed For' on the second occasion. It still didn't arrive, even though the seller said the Royal Mail tracking system showed that it had been signed for by someone sharing the same surname as a guy across the road from me - who denied all knowledge of it. So it could have been a dodgy seller or a dodgy neighbour or a dodgy postman; I never got to the bottom of it).