A sale is an extension of trust. The proof is in the delivery, the fulfillment and the follow-up. When a sales person and the organization they represent deliver exactly as promised, the trust extended by the customer is proven. When promises are proven, satisfied customers become ambassadors for the sales person both internally to their own organization and outside of their organization to a larger circle of influence.
Satisfied customers also become a reference source to future suspects who are invesigating the claims you make. Integrity leads to more business in several ways: satisfied customers buy again, they provide proof statements for use with new suspects, they refer new business opportunities to you, and they become a reference source for new prospects.
Selling is an honorable profession. Unscrupulous sales people who use slick and sleazy practices harm the reputation of all sales people. Most of all, dishonest sales people do themselves a disservice because integrity leads to more business.