The Official RuneScape Forums are a popular place for trading which can be useful for a business.
The forum is a place for discussion on anything RuneScape-related, and a large portion of this is the Marketplace where several players sell and buy things as an alternative to using the Grand Exchange.
Initially, the forums were open to members only, which made it impossible for non-members businesses to use, but during April 2009 a new update allowed high level non-members to use the forums, making them more accessible. To meet the requirements to be high leveled, you need to receive 12.5 million experience total.
The marketplace is divided into several sections (such as ores, logs, runes, flat-packs etc.) and in each of these you can find threads for things like specific trades (eg. someone selling a cat mask) and also larger threads for businesses that have more stock and are less specific.