Why the truth is not in them ??

Do to others as you'd have them do to you. The ones that do not follow this simple rule, are the first to complain when they receive the same treatment they put out.

The clincher of that story is, I told him "we both know this is a very good deal". Some people are not happy unless they can haggle down, no matter how good of an initial bargain. 

autospec, I didn't mean anything against your OP, in case you read it that way, I fully agree with you. Years ago, before internet, I advertised and instrument for sale at FIRM price. A man called and we agreed the price was firm. He came over with his son (whom it was for). Afterward, I got the feeling that he was going to "teach" his son how to do business. They came over, and the man tried to bargain down $60. After his haggling, I reminded him the price was firm, and he's only bargaining $60 less. So his tactic was "yes, we're only talking $60 less, so you should take it". I then said that it was no longer for sale. Then he finally "gave in" at the full price. I reminded him "sorry, but it's no longer for sale, please leave". He even phoned me after they got home, to please sell it at full price. I didn't. I was congenial all along, but keep agreements.