I think one way to handle this would be to have a locked thread called Disputes which consists of single posts added by a mod with no replies. Each post would simply state the username of someone who has been in a dispute and a link to the thread in the rest of the forum where more details can be found i.e. a thread like this.
That way it would not be a blacklist as such which may have legal implications, but still forum members would be able to go to the Disputes thread and scroll through the probably quite small number of posts (most people here appear to do business without any issues) and see if another member is listed. If they are listed there would be a link to the thread describing what has happened, enabling each of us to decide for ourselves whether we want to do business with the person.
That way it would not be a blacklist as such which may have legal implications, but still forum members would be able to go to the Disputes thread and scroll through the probably quite small number of posts (most people here appear to do business without any issues) and see if another member is listed. If they are listed there would be a link to the thread describing what has happened, enabling each of us to decide for ourselves whether we want to do business with the person.