Sales threads can be deleted permanently, but only by moderators. We generally do not delete old threads.
It is understandably irritating to find out everything has been sold out, however, retaining the original thread provides future members with a record of who keeps what plant. This way it is possible to contact the seller in the future and request a rare plant species.
Also, all original posters have the ability to edit their threads in the for sale section and update the status of the thread in the first post. The issue you are describing is a matter of people maintaining their threads inefficiently.