Pretty much the only time I would list potion ingredients is if the players were looking to create a specific potion/potions to achieve 'special' effects. By that I mean that the story plot line revolved around, or at least heavily involved, getting the potion created. In which case determining what 'special' ingredients are needed and then actually obtaining them become part of the story line.
To do things otherwise, when the potion is a rather 'mundane' potion, requiring players to list what the ingredients are and forcing them to go shopping for them would be about as interesting to most players as making them actually go shopping. It would be a chore and not a fun one. The only possible time I might make an exception was if the player wanted to create a new, specific potion in game and had already used up their allotment of potion slots. In that instance, I might allow a player to make some Lore rolls to determine what they might need to get, or have the players make up some rare ingredients, and then have them in game try to get the materials to make an 'extra' potion beyond the normal number of allowed by the number of potion slots.
Given that I don't normally anticipate players fully utilizing all their potion slots and still needing to have more potions, or that players would really enjoy shopping for potion components, I don't anticipate needing to so all that often. Having players go on quests for potion components on the other hand...
By the way, how does this sound for a potion component for something involving water and cold?
An unfrozen tear shed by Queen Mab...
-Cheers