Everything below is live from the in-game Auction House. To buy or sell, speak to an Auction Hall Guide in any major town — Prontera, Geffen, Payon, Alberta, Aldebaran, Izlude, Morocc, Morocc Ruins, Juno or Lighthalzen — and she will bring you to the Auction Hall. In Prontera she stands just south of the central fountain (158, 192).

Stackable items can be auctioned in bulk, and you don't have to take the whole stack: buy any quantity you want at the listed unit price.