Mr. Roberts, the country merchant, first makes a splash by being the first guy to display belief in—and not just compassion for—Guinea. It's kind of a big deal:
"Yes, my poor fellow I have confidence in you," now exclaimed the country merchant before named... "And here, here is some proof of my trust," with which, tucking his umbrella under his arm, and diving down his hand into his pocket, he fished forth a purse, and, accidentally, along with it, his business card, which, unobserved, dropped to the deck. "Here, here, my poor fellow," he continued, extending a half dollar. (3, 68)
While other characters might feel bad for Guinea but still worry they're being hoodwinked, the country merchant is willing to wait for evidence to the contrary. You're innocent until proven guilty in Mr. Roberts's eyes. Everyone else kind of just assumes that all people are guilty. (And they're kind of right.)
Mr. Roberts's generosity of spirit doesn't save him, though: it's a matter of interpretation, but it seems that he ends up dishing out some dough to more than one con artist. So it's likely that his trust has been misplaced at least a few times.
On the other hand, there's no real evidence that Guinea was faking anything, so maybe Mr. Roberts' trust and empathy are good things at least some of the time.