USPS money order or credit card transaction only. Avoid paypal because they have no guarantee that you will get your money refunded and if they find out it was a firearms related purchase you will have your account locked and never get your money back.
With gunbroker, auctionarms or gunsamerica and other internet gun sites, I ask for tons of photos and if they can't or are unwilling then I pass on the deal. When I do purchase gun I only send USPS moneyorder or a credit card. With USPS money orders you can track it and the person that cashes it at the post office must provide proof of identity. If they take your money and run you have their address to give to the police and it is wire fraud, and several other federal offenses that will have not only the local PD and post office on his tail, but you can file a complaint with the FBI and treasury (had to do this once on a shiloh sharps custom grade rifle that the guy grossly exagerated and submitted false photos. Got the money back after the local FBI folks and ATF made a few visits to him.
With a CC you need to watch out for what CC vendor you have. Citi is suddenly anti gun and will close your accounts and put a hold on your funds (they almost ran CDNN out of business). But when a gun is either not received or grossly mis represented then you can file a claim with the CC company and the recipients PD.
Now you only have so many days to file claims with a CC company or the post office, so know your regulations before completing the deal.
Gunbroker has a good buyers section that details all the do's and don'ts of firearms purchases online and also a shipping directory.
ONe thing I hate is an FFL or non-FFL that likes to make up regulations. I recently won a really nice springfield 03A3 on gunbroker and the guy (non FFL) said that he could only ship UPS or fedex and wanted to ship only to a 01 FFL (I have a 03 C&R FFL). After I sent him the gunbroker info on the ATF rules and sent him the BATF website links and regs and showed that the gun was in fact on the C&R list (you can download the C&R catalog from ATF) he still would not budge. He even said that he spoke with the local BATF and they said he was right. I politely asked him for their number and he gave me the run around and never gave me a number or the agent he spoke with. AFter this call I told him I would not be following through with the order and instead filing a complaint with gunbroker. Well the deal finally went though and he sent the gun USPS (with tracking) and accepted my C&R FFL.
Know the regs (BATF and financial) before selling or buying online!!!!! That way you won't get burned or look like a dumbass.
Lifetime member VFW and NRA
"Quando Omni Flunkus Moritati" (when all else fails play dead) -Red Green