No sadly we haven't found a way to do this yet. If you happen to find a way on meta.discourse.com, please let us know! I agree that it be better to leave them open, if we could hide them from the latest topic view.
Maybe... but then still we'd have to be very strict to keep topics from going off-topic. I have seen fora where people start discussing if the price is right, or discussing alternative products that would be a better buy etc.
I think the market should be simple; you should only react if you are genuinely interested in buying or selling so not to waste the other persons time. Probably 90% of these questions and answers are only relevant to the people involved in the transaction.
Also, if I look at other 2nd hand markets online, there is almost never a public discussion section.
I think this can very well be achieved, and is best achieved in a direct conversation between persons.
Also here I really miss to see the need for this third person involvement. If you want to sell your Fairphone 1, maybe it is a good idea to first have a look in the section wanted if there is someone who wants one. And the other way around.
Sorry if I am not understanding what you mean. And I think that we could test a more open response system as soon as we find a way to hide them from the 'latest posts' stream.