+1
Fully supporting this, there's no need for the non-upgraded users to have access to Marketplace.
The only edge-case I find for a legitimate, non-upgraded user to be able to send messages in the shoutbox marketplace tab would be the following:
User registers and buys 1 month of premium.
User posts a thread under the Marketplace section.
User's premium expires.
User keeps promoting their already posted thread in the forum's shoutbox.
If this behavior is intended to be kept as is, I'd say add a check to verify that the user has purchased at least one thing on-site; be it through them having access to a specific Usergroup or through the Award (although that can be traded if I'm not mistaken)