I get the feeling that The Powers That Be would like to see more use of the auctions (notwithstanding the current issues with very large numbers of simultaneous auction postings).
What better way to do so than by rewarding their use with achievements.
Obvious achievements could be for value ("bought [or sold?] an item for over X currants at auction"), posting ("posted more than X items at auction"), buying ("bought more than X items at auction") but there are also possibilities like "had more than X items on auction simultaneously" or "bought more than X items at auction in a single day" or cumulative value ("earned more than X currants from the auction house").