Maybe its bacause this option, by if I try to buy them one by one it still doesn't work. But I discovered, that sometimes it works when you buy ALL the potions from the merchant (if he has 100 helath potions, you got to buy exactly 100). I did so several times, but in most cases there is no way to buy potions. It is good idea to buy herbs from merchants (buying not-food category works perfectly, I think) and then go to alchemist. But it isn't very efficient way...
