PayPal is famous for refunding purchase monies to buyers who claim the merchandise was never received, not as described, or something similar. This is with certified delivery records, photos on the website, whatever. The seller never gets his goods back, and loses the funds to PayPal. This is even without their firearms policy which will leave you holding the bag no matter what.
Just get a check and hold for clearance of funds. That's about 10 days.
The buyer may be perfectly legitimate, I use PayPal quite a bit, it's convenient and I'm honest, and internet-savvy, able to scope out a legitimate seller from the dross, but if you're not comfortable, don't do it. Just tell him PayPal has a no firearms policy and you won't cross it.