When customers buy a product from a vendor the affiliate who directed them to the product receives a commission based on the sales price of the product.
The problem as to wether the affiliate should start with a mass market or a niche market is very common. There are two main arguments the mass marketeer reaches a larger audience and hence can expect a greater sales volume. But a niche by definition should be easier to conquer and so making it easier to become a major expert in that field.
It is important to understand the differences between the two in order to answer this question for yourself. The niche marketeer is attempting to specialise in a smaller market one where the small sales volume would not be attractive to a larger player, he may have a prior interest in this field and become an expert in it. Whereas the mass marketeer is providing a number of different products, not particularly specialising in any one but trying to appeal to a wide range of tastes and hopefully win on sales volume.