If you are a product sales guy, you are NOT selling your product. You are solving a pain-point of a customer.
You need to know:
What are the pain-points the customer have. Know the pain points of the target market overall. Study the pain points of a specific customer you are meeting.
Which of these problems is your product solving. Know your product thoroughly. If it is a technology product, you need to get a first hand experience of the product and features.
What configuration options are available that the customers can vary. What all integration are supported so that you can offer options to integrate with existing market solutions.
Your product deep-down. A lot more than any of your prospective customer know from outside.
Only when you know your product, you can offer solution to the problem of a customer rather than selling a product. Even though it’s just price differentiation.