Ales,
I havent had a lot of luck avoiding personality disordered people (and another one that is a real tough one: borderlines). For a number of reasons, they arent avoidable. N's can be totally charming when they are interviewing you for a job. (and very charming when you are interviewing them). People who have good boundaries in place are probably not N's, but they may not be as initially attractive--to anyone, not just co-N's.
I have had a lot more luck remembering that there are concentric circles to relationships. If you picture yourself as the center of a circle with concentric circles around the centerpoint, and that someone you have just met--N or non-N--belongs in the outer circle. No one, N or non-N, gets moved to a closer circle until there is a reason to. And you go ahead and interact with them in a friendly normal way--always kinda aware of where they are in the circle.
Pay attention to how they relate to you and to other people (their family, friends, co-workers, employees, salespeople), how they react to situations (car accidents, illnesses, parties, big messes, projects, boredom). You dont write people off, or avoid them based on what you see, you simply make your decisions about whether they get any closer to you personally based on what you see.
I also give myself a lot of credit for being able to recover from being hurt. If I know that life comes with bumps and bruises, and I know that I am strong and resilient enough to not be decimated by them, then I can enjoy people and risk being wrong about them (for awhile). Its disappointing when they arent what I thought, or hoped. But I will recover.
CB