Haven't had anything crazier than, "I can get it for less at" emails. If I reply I'll usually send them a permission slip to purchase it from the other place at a cheaper price.
A few years ago our hot tub shorted out due to a leak. It welded the lugs together on the board and I wasn't paying for another board. The tub was here when we bought the house and it was a pain. Knowing how many hot tubs get listed in the free section, I knew it would sit for weeks if not longer. Out came the sawzall and I chopped it into 1' pieces. Piled it all up, took pics and listed it. The ad easily got more responses than any other I've posted. Most were LOL responses. A few wanted me to call them ASAP. I replied to each that I couldn't get to the phone right away, but call me in a bit. I gave each of them one of the other numbers. I doubt they called each other, but it did amuse my mind for a few minutes thinking how that might have played out. Then it happened, a profanity filled 'you so dumb' email. The poor sap used an email address with his real name in it. Quick google search found the guy on a family owned construction company bio page, complete with email and general family info. I responded with "you think your wife, Denise, would be proud that you're trolling ads on craigslist?" and he went nuts wondering how I knew who he was. It went back and forth a couple times, then the ad got flagged and I quit.