Hoping to generate some ideas on a guild store (actual store).
I believe it should be restricted to guild tagged potions. Namely composing. (Invented/Cave potions too?) I feel this will streamline the logistics on getting potions from the big composers to guild members.
Basically there is a secondary storage list on the guild page. The seller sets the price of the potion and it is stored in the guild (store). The number of storage spaces (same/independent?) of current guild storage spaces. This would run parallel to the current store (free potions offered for hunting would reside there, personal use are purchased).
Also possibly a way for 'traveling' composers to store goods there as well? Approved by guild leaders of course. They would have specific access to the guilds page to store goods.