In the early days of Manifold, people made these sorts of markets about their profit numbers. This was manipulated, people got upset, and a norm was instituted against doing that. (No actual rule though, though the general "don't exploit loopholes" might cover it.)

Now people are making markets on their league standings, which are effectivly the same thing. The rule against blatant profit manipulation (i.e. create a market, bet against a friend or alt account, resolve in your favor, and manalink them their losses) has kept that from happening, but less blatant manipulation still happens all the time.

One (partial) solution to this would be to ban markets that have someone's profit as part of their resolution criteria. This isn't a perfect solution, since some people would still want to manipulate markets in order to get a better league ranking, but it would significantly lessen the profit incentive to do so. (And since manipulating profits in this manner generally costs mana, it should heavily cut down on the amount of it that occurs.)

If something kinda like a ban occurs, I'll resolve to PROB.