Welcome to Colorado.
If you click the menu item at the top for Forum, you will get a list of the individual forums.
Scroll down to Trading Post and you will see the different categories of items for sale.
There is a waiting period before you can post in those.
There are sticky posts at the beginning of each forum that give great advice on the rules and recommendations for posting pictures, etc.
While you are patiently waiting before posting ads, you can read and participate in the other forums.
Glad you found Colorado AR-15 Shooters