I have psychological issues with negotiating. If a job offers me 10k less than what I think is fair market value for my skills, they are probably cheap in other ways. If I negotiate them into a higher figure, they will probably find ways to delay raises or expect a comparable amount of overtime. Usually I have a good idea what the job is worth, whether I'm going to take a month or a year to get up to speed, how I compare to average candidates. During the recession I reapplied for a mediocre federal job that I had left for better pay, and they thought they could offer me less than what I had earned when i left. I know my competition, there werent other workers with that specific work experience and with the credentials to be legally hired, even in the recession. Either these managers don't appreciate my value or they want something for nothing. Oh, right, that's why I left the first time.
From am employers point of view, if you ask for 20k more, they can negotiate you down but you might do a lower quality job. I certainly had times when managers would say I had teaching experince and would I mind mentoring of the younger folks paid at my level. Um, sure, I'll do your job when you pay me accordingly.
I recently took an apartment from a friend without doing research into market value. A month later, I realize I'm probably overpaying by 10-20%, and I have to decide if living with a friend is worth paying more, and it might be. But aside from hurting her feelings in asking for a rent drop, I'm guessing she will be less generous with letting me use the garage or living room. I'm more likely to move out than to ask to adjust her rent.