There's a fe things you can do;
If the person is a dealer you could post an inquiry on Tom Bower's board of inquiry. Even if its a private seller post the inquiry anyway. The people on that board will let you know if he's a sh*tbird or a good guy. Here's the web address:
http://subguns.biggerhammer.net/inquiry.cgi
Call the guy and see if he's made any other transactions and, if possible, get references. If anything, you can get a 'feel' for the guy just by talking to him.
As far as payment is concerned your best bet. if possible, is to use a credit card. You could always dispute the charges if things go the wayside. If that's not possible us a Postal Money Order. You won't necessarily get your money back but you could get him for mail fraud (a federal offense) if things don't work out.
One other thing you can do is get him to ship the rifle through his FFL dealer. Get the FFL dealer's location and phone number and call them to confirm the seller has dropped off the rifle and it is as advertised. Then you could send the payment to the seller. Have him agree to this ahead of time, if he's legit he should be more than cooperative.
Hope this helps, Good Luck..