The marketplace is a service in which you can buy and sell games online. You can list games for sale and buy games others have for sale. Note: You are not buying games directly from BoardGameGeek, but instead from an individual Seller.
The Marketplace Module is shown by default on the BGG Front Page. You can also access the Marketplace from the Bazaar tab in the menus or go directly to http://boardgamegeek.com/geekmarket