The Merchant Outbox is accessed from the top menu bar, Avatar → Merchant Outbox.
A window like the one on the left will open, and will attempt to connect to your account on the SL Marketplace. Once it has connected, you may drag items you wish to sell, from your inventory window to the outbox.
A folder will be created in the outbox, for each item. Once ready to send to the marketplace, click the Send to Marketplace button.
For more information, as well as know issues with the marketplace itself, please refer to our Direct Delivery FAQ.