My personal guidelines for any Internet purchase are: 1- Must have a brick and mortar store, 2- Must accept American Express credit cards, 3- Must offer a reasonable inspection return time.
I've sold quite a few guns on Auction Arms and Gunbroker. I don't make a living doing so as my screen name probably implies. By the way, it's the same name I use in my auctions if you'd care to check my personal feedback. I sell my guns, sometimes at a loss, to enhance, weed out, or simply goof around with my collection. Usually I'm lucky to break even but that's just the way it goes. The only gun I ever had returned was a pre-64 Model 70 in .375 H&H magnum. I posted in CAPITAL LETTERS and pictures that the stock had a professionally repaired crack around the tang and then the moron who bought it upon receipt complained that, "it's got a crack behind the tang". I had him return the gun, refunded his money and shipping, and sold the rifle for $400 more the next day with the "buy it now" option. The professional guide from Montana who then bought it was delighted.
I recently sold a 28 gauge Wingmaster Light Weight in near mint condition and got $425 for it. It had two boxes of AA's through it... Pretty wood too... Then there was the circa 1974 Model 14 Smith new in the box that I got $375 for 10 months ago. Yes, target hammer, target trigger, Goncolo Alves target grips... get the picture?
I guess the reason for my my rambling is that: 1) not every private seller is a crook, 2) if you gotta pay by credit card you probably can't afford the gun in the first place, 3) every website that I know of requires a 3 day inspection period for ALL sellers, and 4) screwing around with a U. S. Postal Money Order can put you in a world of hurt.
Yes, there are dishonest people in this world. I know one guy that I sold three rifles to in Alabama. He was shocked when he got the third gun prior to his sending me his payment. I knew he was good for the money so why should he have to wait and I have to make another trip to UPS when I was in the neighborhood anyway. His money order arrived three days later. I've just personally found very few folks in the shooting community to be unscrupulous and most of those who are are dealers. The crooks don't stay around very long and I personally believe there is a special hot corner of hell reserved for them. Just keep in mind that you're paying for that "brick and mortar", AMEX fees, and profit margins next time you buy.