I have bought a lot of top shelf glass at a good deal. Pulled them off EBay, gunbroker, forums, Craigslist etc etc. I've only passed on one, met the guy in person and he was jittery as heck and you could tell something was up, there's been a couple more that never went anywhere after asking a few questions, it was obvious it wasn't for real. I think your wise to get the serial number. Ask him for a few more pics, ask him about it's history, ask him why he is selling, ask him if he has any references on EBay, forums or wherever he may have a track record. If you feel like it's for real meet in a public place, I like having someone with me when I do that. If along the way you get bad vibes then follow your gut, let it go but you never know when someone really needs cash or what their circumstances may be.