alt Hacker News> it's probably for the best that your counterparty doesn't also control the platform
But doesn't Polymarket et al own their own platforms, just like Betfair owns their platform? There is no P2P going on, even if some markets seem to advertise themselves as such so again, seems like the same just minus gambling regulations?
As far as I know it is "P2P", a continuous double sided auction like any proper exchange. If there's something fishy going on with the order book I do not personally know about it. (I don't think there is but I'm also not going to stick my neck out for them.)
https://docs.polymarket.com/concepts/prices-orderbook
https://help.kalshi.com/en/articles/13823828-the-orderbook
Tl;Dr there's an order book with prices set by supply and demand. So you're at the whims of the market rather than the whims of someone who is your broker, exchange, clearinghouse, and taking the other side of your trade. I don't know much about Polymarket but iirc Kalshi is only your broker and exchange, so dealing with them is materially less risky in my mind.