[joosters]

Bookies have implemented this. Betting exchanges have existed for over a decade now, you can lay off your bets to lock in a profit if the odds have moved. Even 'traditional' bookies offer various forms of cashing out a bet.

[coralreef]

Could you name some of these exchanges? One problem is that they may not service certain jurisdictions as well.

[kluge]

Betfair is the biggest one.