You want to use the STAR method here. Discuss not only what the sale was, but also how you were able to make the sale. Discuss what skills you used, and how the sale affected your work in the future. Discuss things like using communication or relationship-building skills, the specifics of the sale, and what ways you think it affected your approach to sales in your work after that. It also doesn't have to be your most financially successful sale, but it might be a sale to a particularly difficult customer, your first sale in the industry, or one that really changed the scope or approach of your work.