How to keep a customer list current all the time

Hello wonderful people ,

So. 

• Each month I receive a file of sales by customer , simple file with customer name in column a and sales in B 
• I would like to build a master file looking at rolling 12 month sales by customer. I have a file for all of 2019 by customer by month so that I can start this from Jan 2020
• My challenge is , as the months go by NEW customer will start to be sold to . How can I make sure that my master list constantly updates itself with any new arrivals 
• To be clear customer EZPEAZ starts to sell in April . Who I get the sales file for that month and look-up into the master file it won't see the EZPEAZ as they never existed before 

Undoubtedly simple , I await your collective brainpower :>)