In a general sense, it helps if you participate in conversations on the board. As you'll see, I don't have a high reputation count, but there was no problem in setting up and executing on the trades I wanted within a few weeks of joining.
If your only interactions on the site are for trading...
Joking aside, I think it's the size of the community.
In a small, tight knit community a bad reputation means that pretty soon, a bad actor ends up with nobody to deal with or talk to. He's either ripped everyone off, or he's ripped enough people off (or flaked on them) that everyone will know...