My method. First, find out what FFL is shipping the gun. This is not required unless required by you.
First, find out everything you can about the seller from their username and email. This will also vet out the gun a bit....I.e. If he is complaining about how their 1911 doesn't feed at all, I wouldn't buy a 1911 from them!
Second, require the FFL. Check the FFL out by Google'ing their name and reading about them. Call them. Tell them your plan is to send the owner a money order when the FFL has the gun in possession. Then the FFL is to ship the gun to you insured with tracking. If the FFL agreed to this and you don't get the gun, call him and explain the situation. If he gets you a gun or money, you're good. If not, call the ATF and explain how the FFL stole your gun. Basically, no FFL will be mixed up in a scheme that could have them answering to the ATF or risking their FFL. They can be held responsible.
Third, use a USPS money order. If lost or claimed lost, there is some recourse, I believe.
Forth, pre-arrange a 3 day inspection period without firing on your end. If you contact the owner accepting the gun or three days after delivery, the deal is done.
While this certainly isn't bulletproof, as a buyer, money moves first. It is the standard. That said, protect your money, protect the gun and protect from shipping theft. The FFL on the other end makes this almost a sure thing.
Add in Skans advice. It looks pretty solid, especially in looking out for common scams.
Now, pool cues, prepare to get screwed out of your money! The pool gods owe me $850 cash or $1850 in cues! One guy died to get out of making my cue! The other, well....he just dropped off the face of the earth!