The best way to avoid scammers and rip off artists like Martyn is simple--research them!
Google their callsigns.
Google their businesses.
Yeah, that's a good method.
I just plain refuse to buy used gear unless
it is being sold locally and I can out my hands on
it, and do my own tests. Look the seller in the eye,
ask pointed questions, sees if he hems, haws or
prevaricates. "You told me you were the only owner,
but you don't have the receipt, manual, or original
boxes for a rig you supposedly bought a year ago?"
But then I don't buy much used gear, live in an urban
area, nor look for esoteric gear that is hard to find.
Next year I will be restoring a 1937 Philco floor
console radio, Model 37-650.