[CrazyStat]

Polymarket charges “taker” fees (people removing liquidity by matching listed orders) on most markets. Geopolitics markets are exempt. A portion of the collected fees then get redistributed to “makers” (people who provide liquidity by listing orders for others to match). Presumably the rest of these fees make up polymarket’s revenue.

[OJFord]

Which is essentially also providing a platform for making the book for the other platform, on which 'bookie fees' are charged, but Polymarket itself only keeps a certain cut of it, for facilitating but not actually book-making.