An appropriate approach for whoever is applying for a sales job has to be result-oriented, that is: What are your achievements? How have you contributed to your previous job? What are your skills?
Do not forget to be clear about your goals and expectations in the sales field, and how you think your experience and aims meet the company/organization interests. Mention any valuable knowledge you possess (Marketing, Management, Strategies, etc.) and be clear about your sales figures.
Sales careers are maybe the most competitive in the market, there are thousands of people that are searching a job in this category, probably you will have to compete with many of them, so what to do?
Just make your resume as good as you can. You have the cover letter to make it work, try to sell yourself, using clear and effective words and concrete paragraphs, be positive, if you are a good seller then you must have a good sales cover letter. Hiring manager will know your sales effectiveness by reviewing your resume and cover letter.