I see a few people in this thread speaking of poker from a position of ignorance.
The difference between long term winners and losers in poker is NOT bluffing. Bluffing is only one piece of a broader strategy, and bluffing is not a matter of having the proper testicular fortitude. A proper bluffing strategy has a basis in game theory. The goal is to find the optimum bluffing frequency so as not to effectively play your cards face-up.
Tells also play a much more insignificant role than casual observers are led to believe from movies. Case in point: I played online professionally for four years as my sole source of income. There are no physical tells online, but there are betting patterns you can pick up on that when combined with a working knowledge of the odds of certain outcomes can help you narrow an opponent's hand range.
I wouldn't blindly say "poker players are the best traders," but I would say there are certain skills that professional poker players develop that may help them to some degree when making the transition to trading.
Bankroll/risk management would be one.
Another would be learning how to dispassionately analyze your decisions. In both poker and trading, you can be rewarded for making mistakes. You can also be punished for making the right decision. Over the long term, you expect to win more if you consistently make the right decision, but short term variance can always lay waste to the best plans.
In both pursuits, you work with limited information and must make decisions based on the information available at the time.