Betting exchanges charge a commission to bettors. Typically, this is only applied to profitable trades where the bettor accrues a profit. The level of commission varies between 2% and 8% and can be adjusted by the exchange based on the bettor's performance level (with more profitable traders paying higher commissions).
Betfair is the oldest and most popular betting exchange.BETDAQ
BETDAQ is currently owned by bookmaker Ladbrokes, but was formerly independent. It is the major competitor to Betfair.Smarkets
Smarkets is a sports betting exchange. They currently charge a commission of 2% which is at the lower end of the scale.Matchbook
Betting exchange operator.Betting Exchanges
Wikipedia article about betting exchanges.