Some items in the game are limited (ie. you can only buy a certain amount from the vendor, then you have to wait for them to respawn). I've only tested this on single limited items (vendor only has 1) but it's worked for me the last 4-5 times I've done it right, and I assume it would work for the multiple limited items if there's only 1 left.

Basically all you do to setup is talk to the vendor and open up your bags. Then you right-click the item you want to buy, then quickly drag the item to your backpack (both are acceptable ways to purchase an item). If you do it fast enough you will be able to get 2 of them even though the vendor says they only have a limited supply of 1. If you do it too slow it'll say "The item not found". It's also easier if you have large bags because then you dont have to drag the item as far to buy it right after you right-click it.