You are correct that Proof of Postage only records the postcode, which can only be obtained at the the Post Office (for free), when handing over the letter/parcel at the counter. However, you can insist upon the address or the property number to be recorded ─ which requires a 'Certificate of Posting', which needs to be filled in and stamped. You can have one or the other, not both. This absolves RM of any negligence albeit they could clarify this distinction.
Since, this particular scam is originating from a very specific ecosystem; in this case, the responsibility ultimately lies with eBay and to a certain extent with PayPal ─ who are blasé about these scams and have zero intent on solving these issues, they are even happy to front-load the system, so it favours certain types of scammers.
https://business.help.royalmail.com/app/answers/detail/a_id/...