it is ethical to sell a product that is, at best,only mildly effective?

It depends on the circumstances. If, for example, there is no such thing as a really effective product, and you don’t lie and tell people that the product is more effective than it is, then it is okay. For example, there are medications out there for things like weight loss and baldness, but none of them work for everyone, and none of them have spectacular results. As long as they do have some effect, and people aren’t told that they work better than they do (and especially if they don’t keep people from using something more effective), then it is okay. On the other hand, there are people out there who sell “alternative” cancer treatments for which there is no real evidence of effectiveness, other than anecdotal evidence. People who are afraid of the side effects (like hair loss) of conventional treatments see hope in these, and they stop taking things with a much greater chance of helping them in order to take the alternative drugs. To my mind, therefore, selling these alternative products is unethical.

As long as you are honest about it. If you are lying about its effectiveness then it becomes unethical behaviour.