Bulk Buy allows you to buy stackable items in any quantity (even if the merchant sells them in stacks greater than 1) and also allows you to buy multiple stacks at once (including non-stackable items).
When purchasing items that are sold in stacks greater than 1, the left and right buttons on the stack split frame will now decrement and increment by 1 by default. When the split stack modifier (Shift) is held, the left and right buttons will decrement and increment by the stack count instead.
When purchasing items that are sold for non-gold currencies in stacks greater than 1, the AddOn will buy the largest multiple of the stack size sold by the merchant less than the amount entered in the stack split popup.
There is no in-game configuration, but if for some odd reason you need to buy more than 10,000 of an item at once you can edit the MAX_STACK number at the top of the core.lua file to raise the limit.
Bulk Buy was inspired by an AddOn with very similar functionality I used a long time ago whose name I can't remember. I couldn't find this AddOn on WoW Interface or Curse, so I decided to recreate its functionality myself. Although the functionality is borrowed, the code itself is completely my own work.