Is the marketing commission for health products permissible, given the uncertainty of their effectiveness and reliance on the product owner's description and the opinions of the majority of buyers, and is it permissible to promote based solely on the seller's description if no genuine buyer reviews are available?

You are permitted to work in product promotion based on your strong presumption of their benefit, whether through consumer testimonials or the product's credibility. This is because Sharia rulings are built upon prevailing presumptions, and a prevailing presumption takes the place of certainty when certainty is impossible or difficult to ascertain.