If you can research the return policy at NARS Cosmetics before your interview, it's a great idea to refer to their policies when answering this question. If you are responding strictly from instinct, be sure to think about what you would like to hear as a customer while keeping the profits of the company in mind. Affirm with the interviewer that you put a value on customer service.
"I agree that customer service is everything - especially in such a competitive industry. I know from being a customer at NARS Cosmetics that you will take a gently used product back in the first 30 days. I think that is fair. If a customer had used a significant amount of the product before returning, I think it would depend on the reason. For instance, if they had an allergic reaction vs. simply being bored of the color. I will always follow company policy and, if unsure, will call a manager for assistance."
"In this situation, I would follow your company policy and accept the return. From what I understand, NARS Cosmetics accepts returns, no questions asked, for the first 30 days."