Bulk Buy lets you buy vendor items in quantities greater than a single stack using the default stack split frame (shown when you shift-click on a vendor item).
This includes both stackable (e.g. buying 50 Weak Flux, which stacks in lots of 20) and non-stackable items (e.g. buying 5 Mining Picks at once even though they don’t stack).
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.