A Junk Farmer is slang for those individuals that drive around in old beat up pick-up trucks searching for scrap metal, old appliances and just generally junk in general.
These trucks are usually recognized by their extremely poor condition and the insane amounts of "junk" piled in the back, which often requires special rigging and/or improvised side-rails to contain it.
Dude #1: "Don't follow that junk farmer too close or you may end up with a washing machine through your windshield."
Dude #2: "Good call. Those junk farmers are crazy."