I don't really understand why you don't want to sell it face to face through Facebook marketplace. There's almost no risk, you can show the buyer its working and they pay cash. If they Rob you, you report to the police as with anything and insurance should deal with it -you literally can't lose, and to be perfectly honest I don't understand why you are so paranoid. Is life really so degraded that you don't trust other human beings in the UK?
In Finland we have a marketplace with no third party, basically gumtree, and what you advertise you sell for cash upfront through bank transfer or cash in hand. Anything goes wtong you report it to the police. Anyone doing pretty crime who wants a criminal record for 600£, well lol. Their name is directly linked to their bank information and their digital security number, they are pretty easy to track down after by the authorities. To avoid any risk you just sell for cash in hand.
Imo advertise on Facebook and cash in hand- you can prove its working to the buyer too so trust goes both ways.