Player Quest Board
Here's the idea. Its a board that either an adventurer's hall, or a tavern could put up. The Tavern or Guild is responsible for keeping people to their word, and the better tavern's or halls will have better reputations incentivizing people to put their quests there.
The Quests are issued on the board, you select a quest and then its put up on the board. The Tavern/Hall keeps the upfront fee of materials you offer, and takes half of it on completion, the completion is done via a system where both parties click a button and agree that the terms have been met. This is like a valve steam trading system where you both click and then it confirms the transaction.
The rest of the reward in material components is farmed by the issuer and if they fail to meet the amount in the given time frame they fail the transaction and the Tavern/Hall can issue a bounty.
The tavern can elect to pay in game cash or resources for the rewards of the bounty, the bounty could be to kill the player a certain amount of times or what have you as punishment.