This video discusses how GPI Procurement Services maintains its customer relationships by having excellent salespeople. Their main focus is communicating, listening, knowing their product, installing trust, and providing the best for their clients.

The salesperson in this video notes that expanding business with existing customers is one of the best ways to increase total sales.  Why is this the case?  What are some of the keys (from the video and your own ideas) for increasing the amount of business that you get from existing customers?