Discuss in detail the mechanism of credit card sales?

You might have to ask again. I'm not sure what you need to know so I'll try to explain how it works. You buy an item with a credit card. The clerk swipes the card through a machine. The machine is like a computer. If you have credit available it approves the sale. The purchase is automatically charged to your credit account. If you had $500 worth of credit, you now have less credit available. It's all handled electronically. If you are purchasing an item that costs over $250 the clerk sometimes has to phone a number to get an authorization code. The company who accepts your credit card has to pay 5% of the amount (of your purchase) to the credit card company. If you have cash & a credit card - you can ask the company to give you a 5% discount for paying cash. Sometimes a company will do it.