Karma is used to rate another member based on the quality of their posts and the reliability of said member (when referring to buying/selling). Keep in mind that somebody with high karma could still easily rip you off, so always check feedback on their profile to be sure.
Some members may have lots of karma but could average out at a low rating. Take my profile for an example... I have a shit load of karma, but started out on this website when i was 12 and thought it was fun to post really stupid stuff. So even though I'm a legitimate buyer/seller and have lots of karma, my overall rating is a 5.6/10.
lol.