Just about everything in civilization works on the honor system.
No armed guards at the local grocery store, no pat down as you leave the library. Most people cross the street without fear of crazed hit and run assassins.
When we move online, though, two things happen. First, word among the black hats spreads fast. One person starts ripping you off and suddenly it's a hundred.
Worse, the ripoffs and bad actions can scale. Sure, only one in a thousand email users is a spammer. But one spammer, aided by a computer, can send a million or more emails in a day. Suddenly, the people who violate the honor system are able to drown out the good guys.
Just like the real world, though, if you spend all your time preparing for and defending against the black hats, you'll never accomplish anything. If you assume that every single interaction online is fraudulent until proven otherwise, people will just move on to the competition.
So, online, you're between a rock and a hard place. The first opportunity is to treat your friends better than ever, because word of mouth online is incredibly powerful. The Net brings significant leverage--you can spread ideas farther and faster.