I've mostly had good deals on here but I had one go pretty bad. I didn't get totally screwed, but it was pretty ridiculous dialogue and had I decided to go that path I probably could have screwed the guy over with a PayPal claim, but I'm not the same kind of person he was and was willing to resolve it without extreme measures.
I don't think a rating system will by any means be a solution to proving out people's reputation, but it gives you some kind of insight into how people conduct themselves. I think that has value.
But on the other hand, I've been on forums with these systems in place, and have seen situations where either:
1) Someone gets screwed but no one cares because user XYZ has been such a peach to the highly vocal forum regulars, who then side against the user claiming they were wronged, without any dog in the hunt or any proof that said user wasn't wronged.
2) User XYZ is a great guy with awesome rep until some life event throws them on slightly hard times and they decide to ride out that good reputation for what it's worth and screw some folks over while everyone still thinks they are so great.
The main issues will be, if it's a forum integrated (software) system, people either will not know how, or not care enough to use it as intended. But it will be a lot cleaner system from an integration viewpoint. Sadly when I see these I usually see them not being used and cluttering up the forum interface.
If it's a thread based system (new sub-forum, with threads naming the individual) it could be a big damn mess, but does allow for more information to be relayed. Jon has a darn clean forum here, the most well organized I've seen lately, if we start begging him for a feature, understand what that means to the overall layout of one of the most clean and efficiently organized boards on the web. True it can be hidden in a relative unobtrusive link, but guess what, you do that and no one uses it.
So, while I think in general it's a good idea, before everyone gets set on thinking one way or the other, consider all the possibilities.