To me the answer is very simple - I will only promote products I will buy myself and that I believe in.
Example - I have never and will never use diet pills of any sort. They might work for some but to me the answer is healthy eating - not pills. Do you think I would be able to convince anybody that they should try it? The answer is a simple NO.
Look at what's available. Promote what you believe in and you will do a great job.
