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.

Study and analyse any product or service that you plan to sell. Consider everything about it: 
Components
Services 
Prices 
Policies 

Know the general features plus both the good and bad features. 
Good features may include such things as:
Quality
Accuracy
Longevity
Durability
Strength
Safety
Reliability
Fair or good pricing
Range available
Availability and ease of supply
Sustainable

Bad features might include such things as:
Delayed availability
High cost 
Poor quality
Short-lived
Outdated design (e.g. secateurs that are not ergonomic)
Negative environmental impacts (e.g. environmental weed, contains high levels of phosphates etc)
Limited capacity

When considering poor/bad features of the product/service, also think of ways to handle questions relating to those features. For instance, you might focus on demonstrating how some positives outweigh some negatives, or on presenting the bright side of a negative feature (without hiding the truth). Also, some features can be considered good or bad depending on the situation in which the product or service is to be used, and according to the different requirements of different people. For example a cheap pair of secateurs may be all the customer needs as they do not prune plants often or it is for irregular use generally. They may not want to spend a lot on something they rarely use. 
Considering the secateurs a good salesperson will:
Offer more than one type
Explain why one is better than another (e.g. better mechanisms, will last years, are easy on the hands etc)
If the customer suggests they don’t want to spend a lot the salesperson will offer a couple of cheaper alternatives that will work well for the customer’s needs – pointing out for example that although with this pair ergonomics wasn’t considered it may not be a problem with irregular use. 

Remember that quick, efficient, facts-only service might be much more valuable to some people than friendliness and chatter, whereas others might be willing to wait longer and accept lower quality product if it is associated with warm, friendly and conversational communications and the product is fine for their application or use. . 
Product knowledge should not be isolated, but should also reflect knowledge of related products or services offered by the business you work in, and also, of those offered by your competitors.