But sports books pitch themselves like brokers, giving fair access to bets. A brokerage-style betting market would be perhaps more fair (or at least, the sharks would take the rubes’ money instead of the casino robbing them) but doesn’t exist.
moreover people go to great lengths to try to avoid trading with winners.
there have been cases where people’s banks refuse to do any more foreign exchange trades with them when it becomes clear they are just arbitraging. it’s exactly analogous to the sports book case.