The problem with this model is the following:

You place a bet, you know the stakes laying on the different outcomes of an event and think your winning chances are good in relation to what you might win IF you win.

But AFTER you placed your bet many other people might place further bets on the same outcome as you. So IF you win then you must share your winnings with more competitors. In a bookmaker scenario that would mean the odds decrease. But the bookmakers assure you the odds you got when placing the bet are fixed and will NOT change when other players place their bets after you. But here you can never know which odds you get because it depends on participants who place their bets later.