For an example, I'm not able to create a new thread in "Domains" section. I got following error message "You must be an established user in order to create new threads in the marketplace."
Hmmm... can you take a screenshot of what you are seeing exactly? Also a screenshot of where you see the "Post New Thread" button you are using would be useful as well.
I managed to get that error message simply by adding "create-thread" to the end of the urls in "Domains" section. So there is no a Post New Thread button at all...
Then create it and specify it is a BUY thread during creating it, the switch is inside the listing creator tool