Auction House and AD

Astral Diamonds are the main currency of the game. You generally get them from dungeons (and a couple of other places like Expeditions in Undermountain, Dungeon Reward Chests, Progression Reward) as rough Astral Diamonds (rAD), which you can refine 100k a day on your Riches page. This is the main thing that limits your progression.

Astral Diamonds are used to buy items on the Tarmalune Trade House (Auction House). And can also be used to buy a couple of things from a couple of vendors throughout the game.

Selling items on the Auction House costs Astral Diamonds: to put anything on sale, you must pay 10% of the price up-front as a Posting Fee (extremely annoying when you are trying to sell something expensive), unless you are VIP rank 8 (VIP is pretty much a premium subscription, which gives tons of benefits, which you can buy in the Zen Market) in which case it's free. If the item sells, you get the Posting Fee back, while you don't if you get your item back before the end of the sale duration (1, 3, or 5 days). Whether you are VIP or not, the Auction House takes 10% of the price on every item sold.

So if you are trying to sell something for 1M AD, and you are not VIP : you pay 100k AD up-front, wait for the item to sell or the sale duration to expire (1, 3, or 5 days), and when the item sells, you get the Posting Fee back (100k) + 900k for the item. If you are VIP rank 8 or higher, you post the item for free, you can remove it and post it again at will for free, and when the item sells, you get 900k AD (90% of the price). This also means that if you buy something for 900k AD and you want to sell it to make a profit, you must sell it for more than 1M (Price / 0.9) to not lose AD, due to the Auction House 10% Fees, regardless of you being VIP or not.