Im in the same predicament as the Questioner. I have Pheonix firestorm and I cant find the Merchant out box. In reply to some of your answers in relation to selling, it takes me back to the same spot and all it tells me is:
Open the MERCHANT OUTBOX window in the Second Life viewer by choosing Me > Merchant Outbox.
Drag items you wish to sell from your inventory into the MERCHANT OUTBOX window.
Organize items into folders in the Merchant outbox if they are meant to be sold together. You do not need to box items in order to sell more than one item in a single listing on the Marketplace.
Click the Send to Marketplace button to upload all items from your Merchant outbox to the Marketplace website.
Im still stumped to where this tab is!!!