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.