I have been thinking of decentralised betting too.

A dark pool to circumvent exorbitant Betfair fees or to provide exchange betting in jurisdictions that have outlawed it or forced exchanges to leave due to tax regimes.

Betfair is P2P but it is not decentralised. Consequently Betfair can charge commissions.

I am looking for a decentralised P2P exchange with no commissions.

How that is achieved, I don't know. It would mean that prices are located on individual machines. But if you are looking for a certain price to hedge a market then you just post the ask and and let it propagate through the network until some bids it on another machine.

The usual protocol will then write a ticket, confirming the transaction. From there you need a reconcilliation after the event has decided who won the bet and a transfer of funds is made. How this is achieved without 3rd parties, I don't know.