Credit Knife Person

Have you ever met someone carrying a load and selling kitchen knives in the streets? Others want to buy it, but they won't sell it and are only willing to sell it to you. You gave him money but didn't accept it. He told you with a smile that he would collect the money when you can exchange two steamed buns for your wife. If you encounter it, don't think you're getting a bargain, because you will pay more than just a kitchen knife...