Of course they can't have "infinite" amounts of raw materials, they just have so much of it that the PC cannot buy the entire stack. The merchant has presumably restocked his huge stack the next time you come shopping. Elfroot is a herb that grows, it can be harvested over and over and over again (well, not the spawns that the player can take, but in the game world they should).
I also think that metagaming to keep a merchant in game is not something you should be forced to do. It doesn't matter that it's easier to play with a healer or that "really good" players can finish the game only using very few potions (requiring either very specialized tactics or using explots). Metagaming is the bane of immersion.
I guess this mod might help:
http://social.bioware.com/project/934/
although I'm not sure the pocket plane merchant has unlimited supply of Elfroot. I wouldn't use it for anything else than Elfroot in any case, the game can become quite unbalanced if you use the other stuff found in there.
Modifié par Gruntl, 03 décembre 2009 - 12:17 .