Perhaps if you are honest in trading you can be trusted to select other trustworthy members that dont try to game the trust system so they can scam with impunity??
I don't see the link between those two traits. Someone can be 'honest' and 'gullible' (or overly trusting) at the same time. Likewise, someone can be completely dishonest, but be a good judge of character.
Just because you would not cheat someone, that does not necessarily make you any better or worse at judging whether the next guy would or would not cheat someone.