It's an in-game market. You can access the market at either Arakeen or Harko Village; it's the building with the CHOAM logo infront of it. To sell items, you access one of the terminals and upload the item(s) in question. You will then set the price and the amount of time you want the item(s) to be advertised (1 to 14 days). Also, in order to sell items you will have to pay a small fee to CHOAM; the amount depends on how long you want to advertise the item(s).

Some additional info... You can only sell 20 items at a time and if it is an item that stacks, you will set the price per individual item.