I recently went through this same process, finding a gun on here and being unsure about how to go about safely purchasing it. The guys on here are pretty good at giving advice, I took the advice I receive and it went really well.
This is what I worked for me. I emailed back and forth with the seller, getting good pictures and descriptions of the gun. I confirmed what would happen if it needed to be returned, damaged in shipping, what the inspection period was, and who pays what etc… Once we had the deal hammered out I asked for some personal information.
I asked if he belongs to any particular Club, ATA, etc… also what his address and phone number was. (You can then confirm all their information by looking it up online) I provided him with my info, ATA, Club info, Address and Tel#. We then spoke on the phone. Only after confirming he was who he said he was, hammering out the specifics of the deal, returns, etc…and having a good talk with the guy did I decide to proceed. I also had a Postal Money Order made out; it seems a better way than a money order and NEVER send cash.
I worked with other guys before from various sites and was less than satisfied with them. The important thing to remember is that we have way more time than money. If the situation seems fishy or you are not satisfied in any way, Don’t Do It.
Hope this helps!