Using a Postal Money Order gives you the investigative arm of the USPS if something goes wrong and you get ripped off. Since it's a Federal institution the charges could be serious for the offender.
Also, is there any way to get some feedback on the seller from other members of the site? If so, ask a few questions.
Next, if no no feedback is available you are left with your instinct, or common sense. Give the seller a call, have a nice conversation with him on the phone and you may get a read on what type of person he is. If he sounds like a used car salesman or a con artist then opt out of the deal.
Don't make the deal if you your initial reaction to your conversation leaves you wondering if he's legit.
There's nothing else I can think of at this time, but that's how I handle my internet gun purchases from out of state, and thus far all have gone well.
Good luck.