Something like iTrader?

[SlapShot]

This is a plugin that VBulletin has that allows you to leave feedback for a certain transaction on a particular thread. I think this could be achieved on MyBB by modifying the rep. system to only accept rep. from posts taken place in the "marketplace" section of your forum. It would come in handy for forums where members trade/buy/sell stuff and I actually plan to implement a marketplace in one of my sites soon so this would come in handy to help combat scammers. A comment for each rep. would be required as well and it should state the username of the person who left it. Also, the rep. point should only go up/down once for dealing with the same user, so if John does business with Bill twice, they should only be able to exchange one rep. point for that previous transaction, rather than two to prevent fraud.

*Note, I would not be using rep. for any other purposes so it doesn't matter to me if it doesn't work outside of the specified section.

[Lennart]

Interesting idea.
If I do understand it the right way you'd like to get a special rating for each thread which is accessible only for the thread author and the people who do reply. You would like to have positive/neutral/negative voting option wioth a short text message why this voting.

Is this correct?

[SlapShot]

Yes.

However, when user A and user B do business a second, third etc. time, their overall rep. points should NOT increase to prevent two people from making up transactions in order to boost their credibility.

[BluEditor]

[Lennart]

@BluEditor:
site is down

@SlapShot: Added it to my to-do-list.