All members need to have atleast 50 posts in order to create a topic in the Marketplace (Buy / Sell) forums. If are under 50 posts, you may post replies to other members threads but not create a topic. This prevents spammers, and having the member get involved in the community before selling his/her products.
If you notice a problem when you are trying to create a thread in that forum, please see if you have over 50 posts. If you do, than wait atleast 25 minutes for the system to give you access to create a topic in that section.