agree that anything that can make people money Stock, futures, forex manuals....there are a tonne of them out there, but they sell. often the same people will buy several products searching for the holy grail
Try sports and food related niches they are less competitive if you search for the sub niches and cling on to a niche that converts then there is no stopping you. They really convert you sales.
You'll need to think logically here.. What are people most craving for in life ? I believe everyone wants wealth (money), to be healthy and loved .. you need to look out for these type of products as they're likely to bring you the highest conversions. Find a niche that has a lot of traffic and target long tail keywords. Al.