1.)Meet face to face at a place of someone who has an ffl. Spend the extra money to have serial number check done. 99% of the time that will tell the guns history stolen or not, or even if that serial number goes to that firearm.
2.) Write down their license plate number. This may not get the person but it will get you close.
3.) Remember any transaction done over the wire is federally regulated, whether it be any phone or modem line. So if ripped off go to the authorities they have no choice but to pursue the issue
4.) If unsure about the person don't do it.
5.) if going in blind get a real address, and send them a letter that has to be signed for (preferably usps) before even doing the deal.
6.) insist that it be done at a gunshop, an up to date ffl can't be disputed. A simple call to that towns , bbb, police or sheriff's department will tell you if it's legitimate or not.
I bought a stolen remington once through e-mail, out $400. The guys name is Timothy Looper. I got back about $290 and he got six months in the pen.