At the bottom right of the site there is a "TOS and Rules" link.
I copied this from that. Basically you have to have 25 posts before you can post in the sale forums.
Classifieds posting rules:
Our for sale/trade forums are offered as a benefit of membership and members must have 25 posts and must have been a member 30 days prior to placing an item in this section.
* Do not post any For Sale (FS) or Want to Buy (WTB) thread outside the classified forums.
* The usage of the sales forums is a privilege for members who contribute in the regular forums.
* Threads which appear to be commercial will be removed.
* A firm asking price must be listed in the thread.
* Sellers are not permitted to ask buyers to send PayPal as gift. Any selling fees should be included in the asking price.
* Sellers are required to have an avatar graphic/image/photo uploaded to their profile.
* Members should consolidate their For Sale threads. If a member wishes to list multiple items at one time, place them in one thread. If multiple threads are created in close proximity, a member may be asked by the staff to combine them or the staff may consolidate them.
* Do not post or repost an ad in more than one section of the classifieds.
* We expect ads placed to be related to the topic of this forum.
* Ads may be removed by our staff at any time for any reason.
* Members must have an established account 30 days or older and an account history that includes 25 posts prior to using our classifieds.
* Posting links, re-directions, or references to external sales or auctions is prohibited.
* All WTB threads must include an intent to purchase or trade. Do not place ads requesting free items or donated items.
* Items listed for sale must be owned by the seller and currently in the seller's possession.
* Posting to bump a thread is permitted once a day.
* Payment must be arranged via private communications.
* Staff may ask at any time for the seller to show proof of ownership.
* Additional selling/buying rules may apply; consult the sticky threads in the sales forums.