Player-made services like this practically require an escrow service, or there is no significant guarantee of payment for work done. The bounty-hunting service should create an escrow account on the relevant server, collect payment from the client (maybe not 100%, but a relevant percentage of the bounty) and transfer this to the relevant hunter upon receipt of proof (with the client themselves providing final payment if <100% is held in escrow). In the event that a contract doesn't go through for one reason or other, or the client wants to withdraw, the service's records should be robust enough that the bounty can be returned.
The problem with escrow is that it requires trust, of course, but that will have to be built, and at least
TheHunter111_1 seems relatively neutral.