MMO and Forex are hell like Niche. They have very high amount of refund rate, that is more then it's conversion rate.
The gravity is what you should NOT look at. Especially if you want to jump on a niche with low competition with other affiliates and cheap keywords. Clickbank is a lot like playing the stock market. The low cap stocks are the ones you need to take a risk on to really get rewards. Sure you can promote "thetruthaboutabs" like a million other affiliates but don't expect it to be your little secret and easy money.
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There are many FOREX product out there but you have the weigh the commissions and refund rates. I still think that the abundance of FOREX products out there is still worth the risk but I have the personal tolerance for that type of risk.