I look for signs in the mechanicals that the gun was a bit more used then the seller is letting on, I also check online that the gun/scope is as new as the seller is making it out to be...
I had a member on the site here ask about a SxS I was looking to trade, I did some looking around and then informed him of all the details I had found... Gun was purchased BNIB by me last fall from a store but was manufactured in the 80's... Probably shot myself in the foot on that one but I ain't gunna lie to make a sale... Besides, now I have a nice piece of communist history in my safe!
I found an excellent deal on a BNIB leupold a few years ago, 5min on my Iphone and I discoverd it was counterfeit and the price was still more then a knockoff could be purchased for... Do your research folks
I have bought a lot of firearms online, I have never been burnt, when I buy a rifle it's most often for parts to build something else.... I'd actually rather buy a gun online and never speak to the seller so I don't have to hear the sob story about how nice the gun is blah blah only to find out he lied.