While regional vendors are a decent way to circumvent excess farming, they usually only carry mats for cooking (for the majority anyway, as far as I know), and you gotta worry about regions that are Beastmen controlled. What I meant was more along the lines of adding some additional vendors either in the guilds or around the major crafting areas that carry more of the basic materials, or have the guild vendors have a much higher stock of items (200+ items from a guild merchant. After 200 the item is out of stock for a day/week/whatever). Could even adjust the prices based on how much is sold or if it sells out, capping out at a server designated price.
For example, I want to buy a bunch of Thundermelons, so I go into the Cooking guild and buy 50(out of 200, just for example) for 40gil a pop. No one else buys any for the rest of that day. The next day they restock the 50 that was bought and the price goes up ever so slightly (say to 44gil per melon). Then lets say no one buys any that day, the following day, the price would revert down slowly back to it's native price. This would reflect the concept of supply and demand.
In the situation that someone comes around and buys out the stock of items and throws them on the AH (Which still occurs anyway), you could then go to the regional vendors or the extra NPCs placed around or even just farm your own materials.
The idea is just to give crafters more options than what is currently available.
This would also let people examine prices of the guild vendors and see if they could turn a profit from the people that like the convenience of the AH, and the people that buy from the AH would have more stock as well. If the guild vendor prices are too high, then they can just go to the normal vendors and try that.
More items flowing through the AH would boost crafting and maybe let low level crafters start crafting and then they would have more items to work with and maybe turn a profit themselves while leveling.