The Greedy Market is a place where you can sell items instantly for fast cash. It gives you a listing of what people want to buy, you can sell them your Gems, Keys, Mythril, Syphon, Nectar, NPC credits or Shaman's Emblems for the price they are offering.
If you want to sell something, just click Sell and enter the number of items you want to sell.
Alternatively, you can place a listing yourself. Select [Add Listing] and you can enter the amount, select the item from a drop down menu, and enter a price per item. You can ask for up to 50,000 of each item, but you will need to have enough gold in your bank to afford at least one of it. Otherwise it will not show.