they are people who farm creds and sell them for real money. usualy it's easy to see who they are. a bunch of people with a name that is allmost the same for all, moving and acting the same as the others.
Credit farmers are a usually a group working together. They will generally be doing missions from a mission terminal in a small player city that's out of the way and not very populated. As said they will generally have similar names.
They are operated by people paid minimum money (usually in China I believe or like places) and the credits they make are sold online.
They will never respond if you talk to them and will disappear real quick if discovered.