If you are to sell something properly, you must know what you are selling, both its strengths and weaknesses.
Only then are you prepared to answer the full range of questions that are likely to be thrown at you by a prospective customer. This product knowledge not only makes you a more effective salesperson; it will add to your credibility as an expert, allow you to consider options in a more informed manner, and will allow you to use your knowledge to direct your sales toward individual needs, attitudes, and interests.
A salesperson must know what they are actually selling, in order to sell it. Knowing the features and specifications of a product is the first step in product sales.