this is the plugin i believe you were using to get the the mining world and whatnot it has been updated for the latest version of bukkit according to the thread.   http://forums.bukkit.org/threads/edit-misc-tp-fun-multiverse-2-2-world-management-bukkitdev.3707/

i think you are posting in the right place. I think potion materials would be a good addition to the shop, although i would have them at a pretty high price because some members run potion shops and adding potion materials to the list of items you can buy from the shop would hurt those people, if the price for the materials was slightly higher than people are selling them for then you don't hurt the player economy and you still have access to them if the player shop goes out of stock. Then again I don't think this will be a big issue once the "auction house" is implemented in the trade district, you should be able to get all the materials you need at that point.