Which process is the one to measure and improve from the customer's perspective the selling process or the buying process?

The Buy-Learning model from best selling author Kevin Davis looks at the selling process from the perspective of the customer

While the concepts of process capability is far advanced in manufacturing, in sales, the process capability is often unknown.

How can the process inputs be measured to create a repeatable and reproducible sales process? Or is sales an art where process performance does not apply?