You can play prediction markets by betting on a swing. E.g. I made a few hundred dollars betting on Harris in 2024 when Trump was at ~65% odds and then selling before the election when it was closer to 50%.
The outcomes are still capped. In that respect, it's more like a derivative market than the stock market. You can trade in and out of options. But the value in the system is tightly defined and, after fees, a net negative-sum game.
"Currently, small fees apply to Crypto and Sports markets.
Starting March 30, 2026, this will expand to include other categories like Finance, Politics, Economics, Culture, Weather, and Tech" [1].
More critically, Polymarket doesn't pay interest on deposits. (Kalshi does.)
[1] https://help.polymarket.com/en/articles/13364478-trading-fee...
Increased insider trading will increase spreads.