What to Sell?
The 3 largest markets by far are, health, wealth, and relationships, these are niches they are markets i.e broad subjects with many sub markets that would then lead you to a niche. An example would be the health market which could be anything from diabetes to gout to irritable bowel syndrome.
Weight loss, could be split down further potential customers may want to lose weight to go on holiday get a 'bikini body', they may want 6 pack abdominals muscles, to lose weight after having a child etc.
A little research in areas like this would reveal if they have profit potential. The same would go for other areas of health, wealth and relationships.
There are 2 methods to starting your research, Amazon magazines is a good place to see what is popular as the magazines need to make money and to do so they need to attract an audience willing to spend money to buy the magazine. The Dummies books website with also help you identify popular markets for much the same reason as the magazine publishers.
The other method is to start at the affiliate network end where the products are actually affiiliated, a great example is clickbank.com
The products are ranked in popularity i.e which are attracting the most interest, the accompanying sales letter and other information provided by the product vendor will inform you of how the product going to help the potential buyer. Further research on Google and related forums and Facebook pages will give you and indication of the size of the market and what peoples wishes or problems actually are.