Sales is the key focus of most commercial and services ventures.
To sell, they must market, must describe their goods or services in meaningful terms and communicate those descriptions to prospective customers and clients.
Selling is the process of accepting demands for goods and services at predetermined prices ... even if the demands are a bit soft.

ExtremeSales is part of the cycle of ExtremeProgramming, ExtremeMarketing, ExtremeSales, ExtremeSupport. 
Done correctly, this cycle fuels a never-ending process of UserStory discovery that creates demands for new ExtremeProgramming, enhanced MarketingMessage and so on.

 -- MartyHeyman
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The most serious issue is the clash of cultures. Some sales organizations are addicted to overpromising. Tremes (ExtremeProgrammer) just say "no". The challenge is to find a style of sales that takes advantage of what an ExtremeTeam can do, while avoiding the kind of behavior that would throw the team off the rails. 

SPIN Selling (SPIN the acronym, not the verb) ISBN:0070511136, High Probability Selling isbn:0963155032, and Start With No ISBN:0609608002 would fit those criteria. Those styles of selling and negotiating are even more extreme than extreme programming. -- StephanBranczyk