You want to discuss here not only that you do enjoy working independently, but why. You can mention things like enjoying the freedom to work in a way that fits your personality, being able to make decisions about how to best sell, and working better with little oversight. Sales is a really independent role and they are likely wanting to make sure you can work on your own without much need for supervisor input. Discuss any ways you've successfully worked independently in the past also and how it's helped you be successful.