Choose you item the merchant only sells once. Fill your inventory completely up (if alchemical, unstack some cadaverine or something, if normal, unstack some flint). Double-click to buy. The message will tell you that "inventory full". Go back the next day. You should notice there are now two items. Each double-click will result in another spawned after 12:00 midnight.This bug works for herbs and alcohol too - simply sell something that they don't normally have and are interested in (like sewant mushrooms) and they will permanently stock it.The bug also works by buying the item properly and then selling it back.Please give it a go and see if it works for you. So far, I haven't heard anyone else mention this bug, so it may be unique to me...