You need to read the rules bud. This is the main one that applies to you.
5) "Members with 50 or less QUALITY posts on top of only being to have one ad at a time can only sell a item equal or less than thier post count ...so if you have N. Cost box call and you have 48 posts ....you have 2 options Start posting to the forum ...or ask $48 for the call ..."
So right now you have 13 posts so you can only sell this or any item for a maximum of $13.00 until you have 100 quality posts.
We want you to stick around and become a valuable member of the forums, but so many new comers were ripping people off that this had to be implemented to protect members. I would also read the rest of the rules in the link I provided below.
http://oldgobbler.com/Forum/index.php/topic,86141.0.html