Any device that is normally plugged into an electrical outlet can be activated by an Arduino-powered relay, according to the rules you set for it.
The possibilities are endless considering the variety of sensors and modules available today.
Some of the most popular sensors are:
Sound Level Sensor
Temperature / Humidity / Atmospheric Pressure Sensor
Tilt or Shock Sensor
Also, wireless communication (Infrared, WiFi, RF, Lora, among others) is widely used and easy to implement allowing to integrate Arduino with business management systems.
Some simple examples for the point of sale can be: presence sensors in rooms for clothes fitting, that trigger messages for promotions or advertising. You can also have intelligent displays that change their communication or price according to weather variants.