Auction is a trade spot. The players can trade tradable items to another players, using the action system. The auction system means that the players must bid to win the item up for auction, or they can simply pay the buyout price to win the auction immediately.
The Ymirheim developers' comment regarding a newly introduced feature:
"Buy items sold by other players or place your own items for sale for a custom price.
You can register up to 20 items (full or partial stack) at a time and set for how long you want it to be listed (up to 20 days). You’re going to receive the money for sold items via mail (or the item itself if it hasn’t been sold during the specified period of time). When you buy an item, it’s going to be delivered to you via mail as well. So make sure you have some free space available in your inbox."
To access the auction, the players have to use a corresponding NPCs that are usually located in towns and cities.
The Auction Market was firstly introduced in a pre CBT2 progress report post.