What is it with some peoples sense of humour?

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My hubby has a van that he wants to sell, so he decided to place an advertisement in the local autotrader, it was for 2 weeks and he paid for it to go in there. In the 2 weeks it has been in there we have had about 4 phone calls for this van, one decided we were too far away for him to come, fair enough. The second one asked if it was big enough to fit elephants and tigers and lions in it, the third one was a women who my hubby thought was a little strange and so said it was sold, so she immediately started being silly and demanding to know who he had sold it too and why would he not sell it to her, and was it because of her colour!! Like he could see her anyway on the end of the phone, and then the fourth one was another bloke who also started being menacing on the phone and making stupid remarks about the van!! Why on earth do people do this? What possesses them to think that this is funny, do they sit there all day and just go through the ads picking out people to annoy and threaten? Have you ever had anything like this happen to you?