At the top of the sub-forum it says "If you are buying or selling an entire website, this is the place to do it", yet the only option available is to list a website for sale.
Users can post a buying thread, it's just restricted because it was causing more problems than it was worth. Was changed a few years ago at the same time all users (not just established) could post sales there. https://forums.digitalpoint.com/threads/changes-to-domains-sites-marketplace.2746733/#post-19125038
Ya, a few years ago some changes were made to that. The buying threads were just causing too many support issues for something we don't make any money from. So it became a premium-only feature, and the selling option was opened up to all users (even new ones) since domain ownership is being verified it wasn't as much of a support issue with cheats and fraud.