I didn't expect this to be such a hot topic.. its not a popularity thing at the other forum... more or less, everyone rates 5 or 1. If you're not a jackass, you tend to maintain a "Popularity" rating of 5... the people that tell it like it is, such as Darque, tend to have a medium rating.. like 3.5 or 4.. since the people who know whats up would give him good rates, while the newbies would fruitlessly throw bad ratings at him.. Keep in mind, you can rate newbies poorly for not searching as well. ;) People who are jackasses, with nothing to contribute quickly fall into the 1 to 2 pit.
Back to the main idea, though, for trades. At the other forum, you must have 25 posts before you can leave feedback... as well you're not allowed to access the Marketplace forum until you have attained this level of recognition.. its true, scammers can make fake accounts, and post under them to up their counts... but often they get caught, because they're just posting stupid stuff to get the count up, or its all in one day. There is always a way to scam.. this just adds greater level of difficulty to conciously scam, and deters a lot of "amateur and accidental (incompetant and lazy) scammers"
If there is no existing script extension for vbulletin, even adding a little icon in the user area that links to an external ratings and comment system would work.. even if it wasn't integrated directly into the boards.
I don't know php or mysql, and couldn't implement it as such.. but I know its a fairly simple thing to setup, an afternoon of programming. I could do it in VB6 easily.. and it could function independantly of the boards entirely with the exception of the icon/link in the user area.. though, I doubt you would run vb apps on your server, heh.
Someone wanna step up to the plate on this to design a php/mysql version? :D
PS: In VB6, it would be deadly slow, and not database based. (in other words, functional, but impractical)