Rather than an "Auction House" what I would really like to have, would be the ability to, from my local merchant to see the inventories of all other merchants on my server. That would often be a big help in determining where to travel for that new high QL rope tool i might need. Because for once maybe one of my neighbours might have it on his merchant, which would mean that I wouldn't have travel as far to buy it :-)
Once we had that ability, next step would be a search function which could be used to for example list all long swords for sale on all the merchants of the server I'm on. The list would have to include stats and prices set for each item as well as the name of the merchant holding it.
For this to be usefull we would have to have maps of the whereabouts of all merchants.