I get douchebag lowball offers all the time when I post things on CL, even when I clearly state in the ad that my price is firm. I get a lot of off-lease C2D notebooks of reasonably decent specs. Ebay/standard pricing seems to be in the $200-$250 range for them, and I'll post them at $120 firm or so, and I always have no problem finding a buyer with cash in hand within minutes of the post going live, which lets me delete the ad, arrange the transaction, and go about my business.
I do feel irritated that there are idiots out there who think that offering me $50 for an item that I'm already selling at a firm price well under market value is a great idea, but that's just the world for ya. I don't even respond to people who can't follow the simple content of a one-paragraph posting on CL. It would be like trying to teach molecular biology to a ferret.