Multiplayer games are a completely different genre than a professional-level API like a stock exchange. You absolutely must trust that a client's business logic is sound; as long as their credentials validate it's simply out of scope for you to do anything other than explicitly execute their instructions, because that's how the traders make money. "Trust the client" doesn't mean state, it means actions - the equivalent reasoning in a game would be disallowing the player from "playing badly", which is nonsensical.