I am trying to upgrade all my orbs. So to save time, I tried to shift-click the orb I want to purchase in the vendor's inventory hoping the game would give me the chance to enter the quantity I want to purchase. But alas, that's not the case. I have also tried ctrl-click and alt-click to no avail. Does anyone know how to do this or is this possible at all?
Currently players are not able to buy multiple amount of the same item from the vendor at once. Instead you are forced to buy them one by one.
This has been reported as an issue in official forums with a lot of players complaining about it, so it's probably going to change in the future, and once it does i will update my answer.
As of Version 0.10.2b you can now use Ctrl + Click to buy items from vendors.