How is "trading" different than "buying"?
I got a gun you want. You buy it. You send me money, before my gun arrives at your FFL's don't you? You trusted me that I would actually send the gun, and you stuck the check in the mail.
So okay, instead of buying it, you are going to trade for it. You send me your gun before mine shows up at your FFL's. You trust that I will actually send you my gun, and you ship yours off to me.
What's the difference?
I can rip you off just as easily, keeping your money and not sending the gun, as I can keeping your gun and not sending mine to you. And actually, if I keep your money, you're kinda hosed, but if I keep your gun you can get the ATF involved, since your gun had to come to me through an FFL.
I'm not a huge "buying online" guy. I've bought, maybe a dozen, all told. Three off auction sites, four or five from legitimate gun dealers, and four or five from classifieds on a bulletin board. In every instance, I sent the money and trusted the seller to send the gun.
I've traded three, online. All three times, after PMs, Emails or phone calls, where we agreed, "Yep, we gonna make this trade", I've boxed up the guns and gone down to FedEx and sent 'em, just like I'd sent money before, trusting that the other guy would send his to me.
Haven't lost out yet.