I think some may be using the terms "insider" and "shark" in different ways. To me:
* Insider: A person who is cheating because they actually know the answer in advance or have direct, non-public, confidential information which materially improves their odds over even domain experts. If caught, they can go to jail. Insider as in "insider trading" not just an "industry insider".
* Superforecaster: A person who has deep domain expertise and/or experience as well as strong research and estimation skills which increase their odds over a naive bettor. This may include historical data or first-hand investigation which is not commonly or easily available to others but has not been obtained illegally.
* Sucker: A person who bets despite having far less than a superforecaster's expertise, experience or knowledge. Probably over-estimates their knowledge while underestimating the degree of relevant knowledge which may be legally obtainable by others.
* Shark: Not really clear to me other than more skilled/knowledgeable than a sucker.
* Insider: A person who is cheating because they actually know the answer in advance or have direct, non-public, confidential information which materially improves their odds over even domain experts. If caught, they can go to jail. Insider as in "insider trading" not just an "industry insider".
* Superforecaster: A person who has deep domain expertise and/or experience as well as strong research and estimation skills which increase their odds over a naive bettor. This may include historical data or first-hand investigation which is not commonly or easily available to others but has not been obtained illegally.
* Sucker: A person who bets despite having far less than a superforecaster's expertise, experience or knowledge. Probably over-estimates their knowledge while underestimating the degree of relevant knowledge which may be legally obtainable by others.
* Shark: Not really clear to me other than more skilled/knowledgeable than a sucker.