This is because a member does not obtain access to the marketplace until two separate requirements have been met.
The first of these is that one needs to have been a member of the forum for 180 days (roughly six months), and the second is that one must have made (at least) 250 posts in the forum since one joined the forum.
Both of these requirements must be met before one is granted access to the marketplace; in your case, you have easily met the time criterion, as you joined the forum a decade ago, nine years longer than the six month requirement for marketplace membership.
However, from what I can see, your post count is still under 250 posts, and you need to make more posts in order to be granted access to the marketplace section of the forum.
Marketplace Rules
Before you can access the Marketplace forums, you need a minimum post count of 250, as displayed in your User Profile, and you must have been a MacRumors member for at least 180 days (about 6 months). Posts in certain forums (e.g., the "Politics, Religion, Social Issues" and "Console Games" forums) do not count toward this total.