If the guy is local...INSIST on meeting and doing the deal face to face (not to be mistaken with doing the deed front to back...LOL). The bit of extra travel and time is well worth it.
If he's local and you can meet, don't pay a cent until you see it in your hands.
Pics can lie. I could take a great photo of a nice GBB that's sitting on a desk...actually, here's one...took 10sec to find one...it'd only take 10 more sec to save it to my own photolibrary....you get the point. (NOTE: The photo was randomly ripped off of google...it's neither a GBB nor is it mine...LOL)
If the guy is ripping you off...doesn't matter if the picture is of the actual item or not. I suppose that you could request that he takes a picture with it on today's newspaper and him holding it with phot ID...(but honestly, I wouldn't do that for anyone who asked...LOL)
If he's not local...then ask someone reputable to have a look at it first (probably at your expense). That way you'll know what's what before you seal the deal.