I would want to ask some questions:
Do you believe that supplementation is a good and helpful part of a healthy lifestyle?
Are you someone who is trained to give a professional recommendation for a nutritional supplement? Do you feel professionally comfortable selling them?
Is this supplement: safe and effective?
Is this product dramatically more expensive than other products available that are of comperable quality?
Many who work in fitness will give you the same answers as all those who have responded to you: nutritional recommendations are outside of the scope of practice of anyone who is not specifically trained in nutrition. On top of that most people trained in either fitness or nutrition would generally be on the side of whole foods, and not in favor of promoting extensive supplementation.
But this question is not just a question of whether you feel that you are the appropriate on to speak to clients on general questions of the importance of a healthy diet. This is about selling a product to make a profit.
Isagenix is a multilevel marketing company. If you go to the web site they prominently display the photos of people who have made millions of dollars selling the stuff. When someone agrees to sell the stuff, the people from whom they buy, on up to the top of the company all make a profit. What does that suggest about the fairness of the cost of that product to the person who is buying it at the end of the chain?
I agree you should look over your contract and see what is required of you, and what are the consequences of your opting out of selling it if you do not feel comfortable doing so.
It sounds like you are in a place where you may need to make some hard decisions. Hopefully your gym will allow you not to participate if you do not feel comfortable doing so. If they are promoting this as a money making project it may mean that you will find yourself having a drop in income, or a smaller rise in income, so if you decide not to sell, you may need to look into other ways to enhance your income.