[Autumn]

Solution seems pretty simple to me. Allow any character on that key to place items into the shop, but only allow them to withdraw items which they had originally listed.

With regard to exploits having been addressed with past changes to shopkeeper mechanics, they have not. You can simply list an item and purchase it on any other character.

That only holds true for non-stackable items (Food, crafting supplies, etc.), the types that do not have character IDs attached to them, equipment cannot be bought from your own character.