What is the most important tool now to be an effective salesman?

Assertiveness.
Self-Awareness.
Empathy.
Problem Solving.
Optimism.

Well it doesn't take one tool to make an effective salesman, it’s an accumulation of many important tools; first of all you need to believe in your product, second you need to Study your competition, it is very important to have good presentation and persuasive skills.
A strong willed negotiator is also a main factor.