If you read the rules when you joined VinylNation, by now you are aware that you must be a registered member for 14 days and have 50 meaninful posts in order to access the trade/sell section of the forum.
Still not showing up? Your post count has not changed and you have been posting?
Simple, clear your cache or hit F5 on your keyboard and you should be able to gain access to the trade/sell section.
If you are posting in the Welcome, Games, Create Your Own and/or Collector's Showcase forum
the post(s) do not count towards your total.
Simple one word posts and responses are also deleted by the moderators. Just use the time before trading and selling to introduce yourself to the rest of the members here and take your time getting to 50. It's not about upping your post count, but more about making your post count so other members can have the opportunity to know you before trading or selling with you.