"If buyer is willing and no lies were told, is it moral to sell something that you know for sure wont do any good or harm?"
Uh. Most of what is being sold fits in that category.
Who gets to decide what is good or harmful? (Excepting harmful as in dangerous, of course, which is already illegal anyway.)
Debunking dodgy stuff is good. It's fun and has the potential to educate people.
And, anything that qualifies as a scam or is not compliant with regulations is illegal. So if you can prove something is illegal to sell, end of the story.
In all other cases, making it a moral matter and going down this slippery lane? I don't think so.
