As a sales representative, your ability to speak to clients has a large impact on the company’s profits. Great interpersonal, listening, and marketing skills are all essential. As sales reps often encounter skeptical or difficult customers, employers will be looking to assess your ability to present a product with ease, counter objections, and bounce back from rejection. Be prepared to give a convincing sales pitch on one of the company's products or role-play a situation in which you have to persuade difficult customer.
The biggest interview question I think anyone should be prepared for is WHY SHOULD WE HIRE YOU AND WHAT IS YOUR GREATEST WEAKNESS.
Don't ever say that you're biggest weakness is you're a perfectionist ... that's corny and it slices your credibility in HALF. Whatever your weakness is make sure it's not a core function of the position you're applying for and make sure you talk about how you were able to improve that immensely in your last position by doing blah blah blah.