The thing to remember is that, in reality, the marketing department crosses over into the entire company. Everyone in your company should be aware of the marketing message, visions, and goals of the company, and should reflect that message in everything they do that is related to the product and your customers. This is referred to as Integrated Marketing Communications, and means that every contact with customers and potential customers, whether it's through advertising, personal contact, or other means, should carry a consistent message about the company and product. In effect, every employee is a sales person, and every employee is a customer service representative. If they give mixed messages about what your business is, then your customers and potential customers will have a distorted picture.
Any bad experience a customer has with your company can affect future sales from that customer, as well as the people they tell about the experience. This bad experience can be anything from a rude receptionist, to poor packaging of a product. There are so many variables that effect whether a potential customer becomes a customer, or a current customer remains a customer, that the marketing department shouldn't always be held accountable for the total success of a product. But in many companies, that is the case.