Rules

• You list in game: at the NPC Auctioneer in Thais or in the phone app. Buy now (fixed price, 30 days) or auction (1 to 7 days, optional buy now price).
• Deposit: listing takes 5% of the listed value (the buy now price, the starting bid of an auction) from the bank. It comes back with the payment when the offer sells, and on its own when the offer ends without a buyer. Cancelling before the end: the deposit is kept.
• The 5% fee is paid by the seller when the offer sells. Unsold, expired and cancelled things go back to the seller's inbox.
• Materials sell as a lot: 1 to 2,000 pieces in one offer (things that do not stack: up to 100). The price and the bids are for the whole lot; the page also shows the price per piece.
• Auction: a bid leaves the bank at once and comes back at once when outbid. The next bid must be 5% higher, at least 1,000 gp more.
• At most 10 offers per character and 20 per account. You cannot buy or bid on offers of your own account.

More on the wiki: Auction House guide, bonus items, how bonuses work. What the materials do: the list of materials.