I never think, or support, the idea of listening on General channel, as it WILL DEFINITELY generate a massive amount of data. I am thinking of a "Privately created" public channel, just like when you use the /channel command, but the channel is hidden in the chat window, so the user will not get annoyed with the info spammed.
I meant, this works on the same basis as Call to Arms. It spams the syntax to a "privately created" public channel. The user does not need to see the channel and they will not. So if two or three bots reply then it's not really a problem at all, because the user can't see it.
Request syntax may be like <Req. ID>|<Keyword>
Response may be <Req. ID>|<NPC Name Matched>|<Coordinates>
Remember that one search may yield multiple result. If user searches "Engineering" they will get about 10 different results of all engineering trainers around the world. Same thing goes when they search for "Gryphon Master" and such.
Another thing is that if all searches and search responses are done through the channel, then everytime somebody search something, other people can collect the data too. That means every search query will help growing the database. Eventually the number of actual 'requests' will be reduced to minimal as everyone have a copy of the DB on their own PC.
As for the server overhead issue, that's pretty much the only thing I am concerned about ever since the idea came up in my head. If more developers can come and discuss on this topic then we might be able to get around it some way.
And lastly, just a quick note here, the AH bot and such will not work on 1.6 - Blizzard will render all automatic movement mods useless. So if the bot is to be kept running, then the AH window must be kept open all the time. After some amount of time, user will go AFK, then get disconnected. If it is to be relied on
casual AH users, the chance that the user WITH THE BOT will be at the AH, more importantly he'll have to be VIEWING the page WITH the requested query. This is just practically impossible. Bot host will need a simple 3rd party program that will move the mouse for him every so often, and as soon as a 3rd party EXE is involved, the ToS is broken, and it's illegal. So I am not very much concerned on spybot/AH bot issue.
If you think "why must the requested AH query on the page the bot is viewing, simple. If you run the bot and are browsing for potions while there is an incoming request for a sword, if you want to response, you will have to SEARCH for that sword ON YOUR AH SCREEN to get the result, right? Imagine how annoying it will be if you are browsing through the result you've searched for your new weapon and suddenly the page changes to results of copper bars 
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Wait I have just realised that the thread is in General Authoring board. I thought I was going to put it in request board, did I click the wrong board?? Or was it moved??