Salespeople need to be able to deal with failure by critically analyzing failed attempts and learning from them. Knowing how you handle failure is as important as understanding how you succeed, and in fact, the two are interdependent. Be honest here, and clearly spell out one of your failures. Start with the goal you were pursuing, and then elaborate on why it was important to you, what did you do to achieve it, why you failed, who was involved, what you learned, and what you would have done differently. Far from damaging your reputation, the hiring manager will appreciate your candor.