Evening by the east Kafra in Prontera. A little off to the side, as if waiting for someone, stands a girl in black. Talk to her and she will pretend not to understand: "Only honest people trade here." But word is already going around Prontera: she can get you what no shop sells. It costs a fortune, but she delivers.
The word
She only deals with those who know the password. Ask about her goods and she will whisper: "The word is found where they pour drinks, and where they guard the gates." The rest is up to you. In the tavern, tongues loosen once a throat is wet, and old soldiers remember more than they let on. The quest "The Girl in Black" goes straight into your journal, and the "Where to go" arrow keeps you from getting lost.
A friend can tell you the password too: the girl does not care where you heard it, only that you say it right. The quest shows up in "Available quests" from level 15, but you can take it on earlier.
Order what you need
Say the right word and she opens the black market window. Find an item by name and say how many you need: up to 10 of one item, up to three orders at a time. Until an order is reviewed, you can withdraw it.
Not everything can be had. She does not deal in shop goods (go to the shop for those), quest items, items that cannot change hands, or loot that only bosses drop (go and fight for that). The staff review every order: they set the price and how many to put up, or turn it down and tell you why. The answer comes by mail.
Nothing stays hidden on this market: the Orders tab shows who ordered what and how it ended.
First come, first served
An approved order stays on the counter for a week, and anyone can buy it, not just the one who ordered it. If you see something you need, don't wait.
You need the password only once, and you don't have to visit the girl for every purchase. The auction window now has a Black market button: browse the counter and buy right from there. Orders are still taken only by the girl.
Everyone pays their own price
Everyone has a personal black market markup, shown at the top of the window. It starts at ×1.00, and from there it is up to you. Your order sells out: the markup drops to ×0.99, ×0.98 and so on, down to ×0.80. You ordered too much and the goods sat on the counter for a week: +0.10 at once. So order what you really need, and the market gets cheaper for you. All market money goes to the world bank, for the server's development.
The market window needs the new client. If the game is running, you will get a "Client update available" message; the next time you start the game, the client updates itself.