Allow 2 people to agree to a amount of coin for an action. Maybe items instead. Terms are included in contract (textbox). Once person who issued contract is satisfied, they accept contract as being complete. If they don't accept it, then can cancel with a reason. If contract is terminated, both people should get to comment on contract. Long-term thought is people can see contract rating of another person.
These contracts could be for work/quest/military action. Really anything. This would help develop a way to utilize in game coins while still keeping the open world/sandbox feel alive.