What is the most important attitude you should have during negotiating with customers?

I believe that all of them are valid as negotiating with customers carried out many aspects of speaking modes but i think the most important is B- Tolerant

Interested, primarily

Exhibit interest and involvement and else required would be covered.

Interested It should always be..

Customers come to us for our assistance and service. We are the best persons who will be available for them to over come their requirements. If we are not interested then we will not be in a position to listen to their needs and fulfill them.  

all of above are required for negotiations

agreed with all

Interested and the customer is always right

All of the above.

you should appear tough so that the other party doesn't feel that he has an upper hand . At the same time , you should be tolerant towards the customer and listen to his side of the story. Only if you are involved in the negotiation will you be able to understand and counter the opposite persons points with valid comments. Being clever will help you in gaining the confidence of the other person. You should also appear interested in the whole deal to take the negotiation in the right direction. 

Tough and clever. Negotiation is win-win process. We should be tough so that the opponent has to struggle to win and we should be clever so that opponent has to feel that he has won.    

Clever

D