I'll be using an optical scanner(barcode scanner) to scan the barcode stuck on the products my customers are buying, and translate it to code the computer can understand.
Since I'm going to have a lot of sales for a long period of time, I'm going to use a hard drive to store the data of my sales. Hard drives typically have a capacity of 1-6 TB. It stores the data in binary and the read heads read or write info as it hover over the spinning disk.
I use a receipt printer to print out my customers receipts. The information of their purchase is transported through a cable interface from the computer. The computer runs a sale software application that forms the calculations of what each customer bought.