Tradeskillmaster (TSM) is a very comprehensive modular integrated package with optional modules covering pretty much everything an auctioneer needs. Everything from managing inventory across multiple characters to buying materials, crafting items, posting auctions and tracking sales data.
Auctioneer - The granddaddy of auction mods. No longer the best for posting auctions but still quite useful for search, sort and database functions.
Ara Broker Money
Ara Broker Tradeskills