When attempting to convince a potential buyer to purchase a product or service, the most important aspect of the interaction is?

To pay attention and understand exactly what the potential buyer requires, before using your skills to convince and close the deal. 

Knowing their needs, telling the benefits and suggesting the right product or service that suits them.

Never make the buyer feel that you're selling only for the sake of selling. Show interest in what they need, and let them know how your product can help that need.