This is an interesting question to me, and I'm curious to see what other users (especially market creators, trustworhy-ish users, Manifold staff members, and people who have been on Manifold for a while) think.

Personally, I think "Yes, as long as there are objective resolution criteria." I don't think there's anything wrong with me trading on some of my own markets like "will CO2 levels reach 425 ppm in 2024" or "is Agent 8 the protagonist of Splatoon 3: Side Order." Those markets resolve based on publicly-available data that I have no control over, so me placing bets on them isn't really any different than any other user placing bets on them. I have no insider knowledge and no way to manipulate the market before it resolves.

But I wouldn't be comfortable trading on markets like "will I think [X] in 2024," "how many moths will I see in my house this June," and so on - either the market resolves entirely based on my discretion or I have insider info that others don't. I'd feel icky trading on those markets - but there's currently no mechanism in place to prevent bad actors from using those markets to enrich themselves at the expense of others.

Yes, as long as there are objective resolution criteria

Yes, as long as there are objective resolution criteria; but only after admin review to make sure the criteria are actually objective

Yes, but only under more strict circumstances