One price per unit, the same for the first unit and the hundredth. There is no volume discount ladder, on purpose: a tier the fee code does not know about is how payouts drift by cents, and cents drift for months before anyone notices.
After payment the buyer's character name and realm, server or league are on the order page. Meet, trade or mail, mark the order delivered. Split large amounts across several trades.
The rate moves — weekly for most games, daily in a fresh league. Edit the price rather than leave it. Realm, faction, version, server and league are on the listing because gold crosses none of them.
Guides by game are at /sell/currency.
Still stuck? A person answers in the chat.