I think with all the payment method variables nowadays the photo request method is a pretty quick and fuss free way to tell how legitimate any 2nd hand model sale is...
When already talking about secure payment methods options or not you are already presuming model or equipment etc. are legit and as advertised correct? And you may have already decided to go ahead with the deal. Before reaching that point I always ask seller for numerous photos of various angles and parts of a sale subject. I think this is about as much fail safe a method as any....

For example- Some time ago (approx 6 months) I responded to an ad here on RCU for a RTF Graupner MB339 as advertised by a fellow named Richard???? supposedly located in London, U.K. Richard was offering me an awesome deal on his ready to fly Graupner MB339 complete with P-160.... he rang me straight away upon my email inquiry and was very anxious to sell me all sorts of 'great stuff'
'Off the bat' I was very skeptical I must say, so I asked for pics showing complete model, turbine installation, underside of model, gear, close up surface detail and radio compartment.
Some days later I receive stock pics of Cermark MB339 and separate stock pic of P-160....
Say no more eh!!!!
I already new it was a scam in this case from the outset as at that point I personally new the Graupner MB339 had not yet hit the market....

Theres always an element of risk involved when purchasing anything site unseen!
So far I have been 99% lucky so far using the simple photo request method when inquiring about any models. As previously mentioned in this thread anyone in the hobby who is legitimate and seriously interested in selling their expensive models will send you numerous photos and on request from any angle with pleasure from my experience....

IMHO Always request plenty of detailed photos before anything else is discussed....

Cheers-