So a guild will be able to buy and sell items to other players.
Now, this NPC will serve the guild as a whole, not individual guild members. And the guild master will take the money, and it is up to him or her to distribute it across the guild, or use it for guild needs.
This NPC is not really difficult to program, but it's a LOT of work! So far I have completed only about 20-30% of the code, and a simple line count reveals 460 lines. Of course, many lines are empty, for formating, some lines contian only a "{" or a "}" or an else.
But still, it is a lot of code. Then there is the extensive testing and debugging, the auctioning system, etc.
I do however believe that such an NPC would make the game much more interesting and would add a new aspect to the social and economical interaction between players.
Later on, if this works well, I can implement some other neat things to help the guilds make the best use of their money, such as perhaps 'bribing' various gods for bonuses.