Even though it's called an auction, there's no "list item" button; you communicate your intentions by typing text like "Offer) [items for sale] / Want) [items you're looking for]"
Example: Offer) DivineDagger / Want) FurCape
Example: Offer) 10,000 G / Want) AssassinsDagger
You can send items and money using the @Send command, and use that to trade.
If you want to make a transaction, it might be a good idea to leave a record of your intention in the auction log.
If you are a victim of fraud or similar offenses, contact the Banishing Order.