A guild "barter" box where you can set up, at the bank (or a stall in a town!) an item for sale, only for guildies, for a price and/or material cost. For example, I use a bag with Blackbolts in it, and in that bag I have three other bags, "1k gold" "or 15 venom sacs" "or 21 snakeweed" that allows guildies to use "the honor system" to get blackbolts at a discount or at a mat cost from me. But it requires them to have full access to the clan vault. This would make it easier to include "new" guildies in these kind of systems without trusting them to your full bank.