There is a orange post at the very top of the webpage which holds your answer...
"We are phasing out the old "threaded" classifieds and switching to the Enthusify system. There are many benefits to the new system, and the developers are very active, so there is more in store.
Please continue to shop for items in the threaded area, but no new items can be listed there. We will announce the details of the "threaded classifieds" shutdown soon. Thanks!"