I'm going to dig this up because of the recent changes in the way the broker works and ask if it isn't possible to make something like this now?
Also, since Blizzard is every bit as strict about Gold Sellers as Sony is, and seems to actually take a more active stance on trying to eliminate them, I have to ask WHY this type of UI mod would be in violation of the EULA? I can't see how it would really be doing anything to cause any problems for anyone and it would be freely available to anyone who wanted to use it, so it's not like it is any different from any other UI mod.
My personal favorite while I was playing WoW was NOT Auctioneer, but a different system called WoWEcon, which actually worked a bit differently. It gathered data, constructed an offline database from all servers, on both a per server basis and a merged basis, then published that data in a freely available mod that you could update as often as you liked and stored the data in a saved varriable file on your hard drive where you could access it whenever you needed to know any type of information about an item such as the average market price, the high and low price and the vendor price. If you want more information about that mod, take a look at this site
http://www.wowecon.com/.