The Marketplace is the place where player auctions for items take place in APB. To access the Marketplace, you must use a Marketplace terminal in the Social District.
The Marketplace has all the controls you would expect from a multiplayer game auction house. The ability to browse different kinds of items such as vehicles and clothing (customized such that it may be interesting to buyers), and the ability to post your own items for sale or auction.
As with all multiplayer game marketplaces, swindlers abound, so be careful what you bid on. Make sure it is what you want and that the price is definitely what you are expecting.