“Relationships” is just a word used to describe the web of give and take that binds people together over time.
A transaction can take place without a relationship of any sort. However, a series of transactions that require choices to be made will slowly build into some sort of relationship, with a brand, a store, a salesperson.
Successful selling in the long term relies on relationships, the transaction is just a score keeping mechanism.
So, next time you sell something to a customer you have not seen before, it would pay you to find out something about them. Ask some polite, human questions, positively reinforce the intelligence of their purchase decision, find out what else you may be able to do for them, and give yourself the opportunity to turn a transaction into the beginning of a relationship.