Whenever someone becomes a service provider (such as BoonEx is for Expertzzz) for commercial transactions, there is a level of responsibility called for --- and I suspect the service provider would be legally responsible for complicity in fraud in many jurisdictions, should it not take reasonable steps to protect its users' rights.

At the very least, the service provider should require the real name and contact details of members engaging in commercial transactions on the site, in order to be see more in a position to effectively interfere in cases of dishonest clientele, if only to freeze their site account and disqualify them from future commercial activity on the site.