In-game auction is usually a server-wide market that is available for every player at all times, though can be restricted to NPC or specific towns if game wants realism.
Typical elements:
Search - The most important part of auction is searching for item type or class.
Add new listing - provides UI with presets(default increment/price/type from previous bids) to list new items
Result page - the items sorted from lowest to highest price.
Listing page of Item X:
Start Price: The owner's set minimum bid price.
Fixed Price: Disallows bid and sets Max Price == Start Price.
Usually equivalent to just selling at low Max Price.
Period: Time for which the auction is active.
Bid price: N currency. Adjusts based on last bid.
See Increment minimum to understand.
Add/Remove bid:UI elements to add/remove bids(Bid Price minimum is used to start a bid) with confirmation.
Stack: for stackables the amount of units listed and their per-unit price.
Max Price:N currency. aka "Buy It Now" price that allows instant buyout - which also ends the auction. Game can have set limits on Max Price that are designed to encourage its use such as x30 list price as maximum Max Price(x30 of last bid can work for shorter auctions ).

Increment Minimum:Percent of last bid(e.g. +3% minimum) or flat +10 currency minimum increment. (Used to prevent small increment spam bids.)