I think the marketplace should be implemented in that way that the user can actually hide the offer when he has bought/sold the FP item. So the visible items are from those who are still waiting for a buy/sale.
To keep it tidy, offers should be hidden automatically after three months (assuming that the user has simply forgotten to hide the offer after buy/sale), the user should be informed and receives a link where he can un-hide the offer (if he still waits for a buyer/seller).
Public answers to offers should be allowed, for questions, to show interest, for linking in "wanted" topics to "offering" topics or for finding the person who has the best price (a. k. a. auction). Only bank details and addresses should be exchanged privately.