What I would add to this is before writing your cover letter I think it is important to write an impactful resume. That document should outline what you have accomplished vs. what you did. Since this is a sales resume I want to know how much sold. With the cover letter you can go into more detail about how you accomplished your sales results. Additionally, you can relate how your sales experience transfers to the position that you are applying for.

Realistically, in this electronic age of too much information, hiring managers and recruiters will look at your resume first and see if you are even worth their time. If you are, then and only then will they read your cover letter.