A simple Discourse plugin that allows for topics in category to show trading buttons. It's meant to be very simple solution for sell/purchase/exchange type of categories.

You just enable the plugin. Allow appropriate buttons in category settings:

And every topic gets those buttons above title (of course you have to be the owner of a topic):

After you press the button you get confirmation whether you would like to continue and if you do continue the topic will be archived and it gets prefix according to the button you pressed:

Feel free to use it of fork it.