If you can post replies than you're all signed up etc. I would say that you have not reached the required criteria for starting new threads yet. The RSLC has had a lot of international spammers in the past starting new threads for penis pills and such, so they have implemented a few securities to stop them. Read thoroughly the section on introduction, I think maybe now you need to introduce yourself before you can post a new thread. Possibly there's a post count before starting a new 'for sale' thread to stop people using it as Gumtree.
Let me know if you're still stuck getting the introduction.