Like many, I think there are two issues here.
His liberation of the nameplate, to preserve it for posterity, is fine in my book, if it was otherwise going to be binned.
But to then flog it knowing or suspecting it was someone else's property? Hmm.
But the rules are vague and contradictory. As a sprog I was allowed to visit the station scrap compound and liberate any aircraft part I could carry. A favourite was Airspeed Indicators, to convert them into clocks. Very lucrative. But if MoD Plod stopped you when going home, you were on your own. Even then, the likely reaction was 'Can you make me one?'
But it does emphasise the complete loss of asset control in MoD. The rules, now widely ignored, require each item in the inventory to have an 'owner' - the Service HQ person who makes materiel and financial provision for the item, and dictates disposition. Even the above scrap. Similarly, there are rules governing loans, be it to or by MoD. MoD is careless in the extreme with its own possessions, so there can be no reasonable expectation of it caring for others'.
I once paid for updating a 'computer system' for a company to update OMEGA software every 18 months. A single 286 computer replaced 14 MoD-owned BBC Micros. The company even had an official archive replicating the BBC's own library, crates of spare parts, and had been authorised by MoD to use the Micros for their computer club, the deal being the club members kept them serviceable. (Not an easy task). In practice, the software was updated at club gatherings, and we weren't charged. When leaving the factory that day, a couple of storemen were waiting with all this old kit. The 'loan' was finished, here, take it back. Nobody in MoD was interested, the kit 'belonged' to me as it was held on my contract, so my problem. I gave it to my daughter's school. MoD's auditors did their annual audit (another old rule), and never blinked.