Furthermore, I also had an idea that they will only purchase so many of a single block before they simply aren't interested in it, and if it is a renewable resource, and they have access to a way to generate it, they'll never purchase that item. So if they already have a 'quarry', they'll never purchase cobblestone, or anything else which is directly crafted from cobble (smooth-stone, stone brick, cobblestone/stone brick stairs/slabs/walls). This keeps you from simply dumping fifty or so stacks of cobble on a village and instantly being able to buy anything you'd want.
Again, everything is still at the 'planning and plotting' stage of development. I think we're going to start with the actual village spawning mechanics and layout improvement first, and see if we can't get that hooked into other mods, before working on villager AI. It seems, at first glance, to be the easiest of the challenges to undertake, and will have something which is able to be released as an alphadev version.