I think it is always worth leaving a negative after a problem. I see your point about retaliation but as TheMagicM said, there is the facility to make follow up comments. Whilst they may seem like a waste of time, if you had a very good record with just 1 or 2 -ve's, then I think people do check up and will be more likely to believe your responses.
e-Bay can be hugely fustrating but ultimately works reasonably well considering the number of users to be satisfied. Of course it is not perfect but it needs people to stand firm for it to have any chance of working. Long time users understand the pitfalls and will be able to spot those who just issue -ve's in retaliation and also spot those whose reputation has been unfairly tarnished.
