There may be confused about what marketing is. Some are trying to answer with the definition that marketing is an activity to offers something to the target market. There is another people that describes marketing as a communication process to provide information on products and services related to the needs and desires of people.
Whatever the definition, actually marketing is simple.
You can ask what people want. Then, you provide what they want. Likewise, if you've got the customers you may ask about the product that has been consumed by your customers and fix it. If you want to grab a market. You may ask what is still less from the product they consume and fill it. The point is to ask for input. Ask what people want , and fill it. Actually it is very simple, is not it?
Many business person who bother to create a new market, or trying to make a product that they think is good. The money will come when you find the market, find people who want something, and provide it for them. Human desire is endless, so it is certainly there is very much niche that we can play. Unlimited and more.
So, if anyone feels not know what to sell. Try asking what is needed by the target market, and began to provide it.