Not too advanced and shouldn’t be very hard to implement.
Right clicking an item at a vendor that is stackable should give you the option to click on Buy X. When you do, you get a input box for a number where you write the number of items you want to buy. This should preferably work with more than 250 items, thus creating several stacks in inventory. Check number of inventory slots, predict number of stacks, either complain about too few slots or take money/karma and give items.
Especially useful for:
Consumable vendors like spy kits, elemental dust etc.
Crafting vendors, especially cooking ingredient vendors.
The alcohol vendors, for the thirst slayer acheievement (you have to double click tens of thousands of times to buy all alcohol).