Bulk Buy is a WoW AddOn that 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.
You can download the AddOn from Curse or WoW Interface.
You can find the latest alpha versions on CurseForge and the source code on GitHub.
The AddOn and its source code are licensed under the terms of the MIT License, detailed in LICENSE.txt