Sites If you are buying or selling an entire website, this is the place to do it. This is for domains with content (not just domains).
Domains Selling a domain or looking for a domain? This is the place. This is not for domain appraisals.
Design Contests This would be the place to hold design contests (logos, t-shirts, icons, business cards, etc).
Advertising This area can be used for advertising (both buying and selling).
Secondhand Licenses Are where you can buy and sell used server software licenses/scripts.
Services If you are looking for (or offering) services, this is the forum to use. This includes job listings, etc.