The sellers have their own fan pages, so all takes is a search. Sure, some of these dealers claim they are only interested in selling their guns through a licensed firearms dealer. But devote a few minutes to looking and you'll soon find someone not only willing to sell you a gun but to do so in a way that avoids any pesky background check. For example:
See? This guy wants to sell his Glock the legit way (via a FFL, or Federal Firearms License holder), but if you throw $20 at him, he'll meet somewhere in Pittsburgh and hand it off to a stranger. See, where most people agree there should be some kind of background checks so that there are more hoops between crazy people/terrorists and owning an AK-47 than just handing over a few hundred dollars, some 40 percent of all gun buyers avoid a background check entirely. That's because private sales between individuals aren't really regulated (particularly "face-to-face" sales), so to make your gun-selling operation legal you just ... don't call yourself a dealer. At that point the only rule is that you can't sell to somebody who you know wouldn't pass a background check (that is, felons, the mentally ill, toddlers, etc.). But you also don't have to ask them.
I had to see this for myself. I logged on to Facebook and found the gun fan pages looked like Tactical Weapons' classified section: you can find a Bushmaster XM15-e2s AK offered for $800, or a Bushmaster M4 "fresh from the box" for $1,200:
One of these.