[GameWizard]

Well there is no modifcation that does exactly what you seek to achieve. However, you can be creative and create your own system.

For example, you can have them register first. Then use the form hack to submit their product. It would send the product information into a thread awaiting moderation. Once approved, you would guide the user to subscription area of your website, they pay the subscription and they enter a special usergroup and their thread activates in the forum ready for review.

That's about as creative as you can get, and initially it wouldn't be automated, but you can definitely create hacks or modify existing ones to allow this to work.