I am less familiar with the path that one needs to take to become an audiologist than I am with entering into the world of sales. I have sold products my entire life (mostly advertising) and I am certain that selling hearing aids is very similar.
To become a successful hearing aid sales person you must know your product inside and out. I recommend reading and studying all the materials you can about the major hearing aid brands. A great resource for this will be the websites of the brands. Websites provide detailed information not only about their products but also about the history of their companies. These details are not only good for selling hearing aids but also to get any type of job in the industry.

Hearing aids are a massive market and this comes along with enormous job opportunities. Start reading all you can about hearing aids as a first step to a career in the field.