The ATF and FBI agent is going to win your confidence and trust, suck you in until you can't spit out the hook. The friendly guy who knows lots about fire arms, always likes to BS, has cash or a fake account inwhich he/ she may pay you to do something legal, then shady, then illegal. The unknowing does the illegal because they want the money and have delt with them before. Sort of like fishing, the fish tests the bait, nibbles a little, puts it in its mouth then bam the hook is set.
Don't buy anything illegal from people you don't know! You never know who is the "real" seller. Some of my family from Michegan have a neighbor who bought several drop in action parts kits to make a fire arm full auto. Guess who came a knocking? You betch the BATF.
Another common approach is to just lurk, monitor messages, e-mails, chats, sort of like a fly on the wall or a sub at sea. When enough evidence is gathered, drop the hammer. Pride gets you into to trouble, people know how to hit the warm and soft spots that get the unwarry talking. Personally, if you don't want the "public" to know about it, DON'T brag about it in public wether its a phone, internet, or on the street corner to strangers.
Asking someone how to make a machine gun, albeit taboo, would not be my first idea of getting others to brag. I would do as somebody above mentioned. "Show me pictures of your ARs" or "I have a Colt bla bla bla, whats yours look like......." Did the three little pigs let the big bad wolf in? Not until he looked like granny!