May I suggest that the following is included with the marketplace rules?
Quote: Questions about the item can be posted within the thread itself. The resulting clarification will be beneficial to other potential buyers.
Sending offers, completing purchases, or discussing pricing must be done through PMs.
Recently, there has been a debate as to whether or not a question regarding an item's condition should be asked privately or publicly.
Some argued that such questions could be detrimental to the sell.
My position is that the clarification of such questions is a service to both seller and all potential buyers. If there is no issue with the item, the seller can clarify and everyone wins. If there is an issue with the item, the seller should be grateful for having it pointed out early rather than causing disappointment later. All potential buyers would benefit, not just the one asking.
I sincerely hope there won't be a solution that only allows the OP to post to a SELL thread. The use of the marketplace is free and that's fantastic for sellers. I don't think there is any significant history of problematic discussions in the marketplace and I believe disallowing any discussions would diminish the value of the marketplace for everyone involved.
Companies like trademe which depend on sellers offering items in order to make a profit need to protect sellers and do everything they can to achieve high selling prices. In contrast, this forum is a community's communication vehicle and it should do everything that a sell is a win:win situation for both seller and buyer. AFAIC, that includes allowing questions about the item.