I think implementing a contract system for high risk server deals would be a good idea. Each person would make two books (each party getting a copy of their own and the other person's), describing the deal and saying they agree with the deal, and if they were to break the contract, they would get some sort of punishment (a ban seems a little harsh, maybe a mute, or implement /jail). Of course people would still be able to do deals without contracts, and not get in trouble when scamming. If someone scams with the contract, the other person could give staff the books, and the person would get in some sort of trouble.