What is the best workflow for a classified forum part ?
Eg when an item is sold/purchased/exchanged should it then be marked as “unlisted” or moved to a trash category?
Or are there plugins?
How to allow a user to unlist it ?
I’d probably use discourse-solved, so that sold items get marked with a checkbox.
And put a link to your Classifieds subforum to show only the unsolved topics, like https://meta.discourse.org/c/support?solved=no
Well, consider this plugin too:
A topic that has exhausted its function (article sold, changed or traded) can be archived by the seller (aka topic’s author).
My plugin archives the topic. I needed the topic to be searchable (for bulk actions) … unlisted (hidden) topics cant be searched.
So people would mark topics as sold/purchased/exchanged and category moderator would bulk-move them under archive category. The easiest way if you want topics to remain present but don’t want them to stay under trade category 
Any new functionality here in newer releases ? 
ここに更新情報があればぜひ見たいです！「機密」カテゴリを実装できた方はいらっしゃいますか？
機密とはどういう意味ですか？
プライベートカテゴリのことですか？グループを作成し、その特定のグループのみにカテゴリへのアクセス権を付与しますか？
つまり、アイテムの売買・交換が行われる場所に関する分類のことですが、このトピックはそのことについてです。