Craigslist is what it is. Most buyers are going to offer less than asking and most sellers are going to ask for more than they'd accept. You can always counter-offer and a lowballer is still a potential buyer. Posting something for sale with "price is firm" or "no lowballers" is a turn off to me. As for the humor, I don't mind it when well done, but if there is a chance you are going to turn a potential buyer off with it, why do it?
Chris