A Point of Sale or Point of Service is generally a machine which calculate the amount of item/services a customer owed to the trader. A customer can pay in cash or via a credit card.
In general following steps involved in a POS.
A Customer puts Order
He/She Pays the amount
Order is served by the Business
It is the most simple Data model of a POS. Product Table holds the information about product. Customer_Type table can tell you type of the customer like a walk-in or a regular customer, a customer with whom you an agreement like that of a groupon. Payment_Type table tell the type of payment like Cash, Credit Cards, a Voucher or may be a coupon. You can extend it as you want. The Orders table is central as everything is related to a order it contains foreign keys to all the tables. All types of reports such as total sales, total orders, items per person, time max sale happen and other can be easily be generated. Off-course it is not perfect but it is simple and covers a normal POS. It is never recommend to complexity things when simple things are efficient and effective.