Everyone has to start somewhere on the PX, but being a new member with no feedback and minimal post count (which are all in the PX) will certainly set off alarm bells with all the scammers here lately.
However, most scammers usually don't pay the fee to post a for sale ad and typically prefer to respond to wtb ads.
Also, if you look up the address on the GAP invoice he posted, the roofline and shape of the house in the Google maps aerial view as well as the pictures of the house on Zillow match the house in the pictures of the rifle sitting on the barricade... That's a lot more believable than some of the crappy pics that scammers here have sent to me in the past where they pull a stock photo of the item from somewhere and hastily photoshop the background.
The pics and invoice address matching tell me it's probably a legit but brand new member and seller... But buyers need to do their due diligence and a phone call is always a good idea.
Another note to the OP... Might want to edit that invoice picture so you're not posting your home address and phone number on a public forum for everyone to see.