Last rack shelf in the store

I hate it when people are rude and they don^t know how to act. Sometimes you can tell right from the beginning that someone is a terrible person that you can^t trust. Am I right or not? Such people are really bringing me down and it is hard for me to understand why they are acting in this ridiculous way. In my opinion it is better when people are living in peace with each other and when they share respect. Some people simply lack culture and there must have been something wrong with their upbringing. I can^t explain it differently and I am sure that you know what I mean. I work in a store selling computer accessories and recently I have understood it. There was only one rack shelf left and one of the customers have noticed it. He asked me whether because of this fact he can get some kind of discount on it. It was really surprising for me that he is asking about it but I have realized that this is the situation on the market usually. He thought that in our store he can argue with me about the price as well. All I can say is that he was wrong. When a product has a price, this price is valid for the whole time. It doesn^t matter that this was the last rack shelf we had. Do you understand it or not? Please tell me what would you do in a situation like this.