Possible Bug with Stacking Items

Possible Bug with Stacking Items

When I try to buy stacked items from vendors in the game, I can't seem to get them to unstack, so if a vendor has three stacked potions for sale, for example, I have to buy all three.  I can't find a way just to select one or two from the stack. Is this a bug, or is there some way to do this that I am not aware of?