I do agree that both of the interpersonal skill and business knowledge are important during negotiation. However, interpersonal skill should definitely weight more since that is the drive that provides the trust to the customer of the product you are selling. Also, providing the technical details is important but with the mix of the interpersonal helps to better communicate about the product. For instance, if you have someone that knows all about the product but doesn’t have the ability to connect to the customer, it is hard to sell the product as an item that customer would benefit.