I've been asking for this for some time now. It really should be this simple.
Member A purchases a widget from Member B
Member B receives funds and releases widget.
Forum invoices member B % listing fee. (there would be no transaction cost) Done.
If member B does not pay said invoice member B looses marketplace privileges. At this point you are still not out anything plus you have not assumed any risk . The widgets my communality will sell will be at a much greater cost than the 10$ hooks sold here and I do not want to assume the risk associated with online transactions.
Let the seller assume the risk (after all they are selling the item) and let me collect a listing fee.