There will never be slaves or people forced to fight.
You make contracts on what type of fight it will be. The combatants sign and agree to first blood, submissions, to the death, first to eat sandwich, etc.
The prizes are whatever you make them.. You can have people fight for a high quality weapon that was crafted or whatever you have. The prize will be part of the contract.
I don't know how the organizing of events would work to be honest, at least when it comes to NPCs. I know we can have sign boards for advertising and notes that npcs can read. I would guess you could post an ad using the ingame communication system that npcs could understand.