Do stockbrokers and professional card players rely on game theory? ?

Maybe intuitional, instinctual game theory

Intuition being underrated

I don't think game theory applies to either situation. Game theory relates to how logical people interact in conflict and cooperation. In poker and legal stock trading, there is no cooperative component, no such thing as a win-win situation. In a completely adversarial situation like poker there are no opportunies for groups to form partnerships. In stock trading, traders working together to manipulate prices is illegal.