Is there a Thumb rule for Sales, Communication & negotiation? If yes what is it?

As a thumb rule for Sales I would say great customer service is the winning formula and always know who is buying from you? What do they buy?  Why they are buying from you and why they have chosen you?

For Communication: Practice make perfect, be clear, concise and a good listener

For Negotiations: Successful negotiations are70% preparation,20% implementation, and10% acting. 

Solution selling course can give the full details - Selling is a mind game these days

There are certain basic sequential steps to be followed which may change depending upon the requirements. Viz: Suspecting / Prospecting ; Need Identification; etc.

Communication plays an important role to understand the need and make the prospective buyer understand your offer or else the gap areas may become the reasons for confusion and loss.

If the understanding of the need by the proposer is clear and the same is of the correlated offer then negotiation sails thru smoothly.

Thus in sales one has to move in sequence with strong sense of responsibility or else falls back to the earlier step to take off once again to succeed.   

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Yes it is basic rule and a first step to communiate to express your skills and followed by the second step - negotiation towards a deal. The result is100% satisfaction for (both). This is the first graf towards a  growth path.