Hi,
There is a multitude of "one to many" chat/ticket/support systems out there for Joomla but, as far as I know, no "many to many" as required here, at least none which is storing the communication in the site.
In the very near future I need a "many to many" system for a completely different project (no shop or market or so). If I find or manage to build a solution, I'll try to remember this thread and let you know. Vice versa, if you come across something, your post here will also be appreciated.