As GTAce mentioned above, you need to be a member for at least 4 days in order to start a thread in the marketplace. The number of posts is irrelevant. You joined the board the 14th of July.

It is the first important note in the rules that you have read (well, supposed to scan through) during the registration process:

"In an effort to reduce problems with new accounts and advertising, new accounts have restricted privileges. They cannot use the Private Message system or create new threads in the Marketplace or certain other forums. After being registered for four days (regardless of post count), these privileges will be raised to that of normal members (being able to send/receive PMs, etc.). "

http://www.talkfreelance.com/rules.php

Hopefully this answers your question.