ExcErr -
1. Yes, a user can (and would in your scenario) have a negative rating if they go below zero.
2. I'm not really sure you would want to restrict it, because I currently have users who are filling out all their trades which include multiple deals with one person. I don't think it makes much sense to say that you could only rate a person once a week or so. If you find a person abusing the system, is that really a person you want on your forum?
I could however add in an admin option to prevent a person from rating other people... that would make more sense than limiting how often they can rate other people.
3. That's a good idea, to expand on it a little more, how about if I make it so you can click on any heading to sort on that column in the popup window? That way you could do name, date, etc... Currently there is no sorting, it just lists them as they were entered in the DB... (So I guess you could say it is by date). But that is also why I color-coded it, so you can pick out the negative feedback easier..