Guess the title says it all. A marketplace with well defined rules for users can be engaged to encourage members to sell their products and service. A method to review the products/services being offered would further improve the quality of the offerings. I have found this to be pretty standard offering across most webmasters' forums.
I must admit a marketplace seems like a good idea, but I would like to see it in the subscribers only area, may seem harsh but the cost of entry stops time wasters and spammers in both areas advertisers and potential buyers,
I could see it being a handy way for webmasters to sell or buy direct advertising or to find reputable SEO, coders or web designers etc.
Somewhere that members could swap (or sell) their services would be great - especially for problems that would only take an expert minutes to solve.
For example, say my website has become very slow recently. I know more about creating website content than I do about creating and maintaining a website, and I have no idea how to improve my site's speed. An expert might take just a few minutes to find the problems and tell me how to fix them. In exchange, I could offer my services as a proofreader etc. Alternatively, the member that helped me could be awarded points that could be used to purchase services from any of the others participating in the service swap.
I would be happy to pay a small fee to WebmasterWorld for the opportunity to swap services with another member.