In DD NPC inventories would reset after about 10 or 15 minutes real time (depending on the NPC). I don't think I ever checked that in BD, but the BF merchants' inventories are regenerated each time you enter the BF, so after you get access in each act you could just enter and exit the BF repeatedly to get more potions (or check for better equipment, etc).
NPCs can decide to hang on to some stuff you sell them, which stay at the top of the inventory list, so if you sell/trade a lot with a particular NPC, you may need to scroll down to check for new stuff. Some people only sell loot to the potion dealer in the BF, so it is easier to check for new equipment with the other BF merchants.
I hoarded potions a bit, and never ran into problems not having enough. The starting level can be a little tough on potions (depending on what you find for random loot, and how much you sleep or just wait to regenerate), but after that things should get better. The healing spell you can learn on the second level (after the arena pit) comes in handy to heal after a fight, and you can find a portable bedroll to sleep wherever you wish (as long as there are no hostile creatures nearby).
I also kept alchemy plants, but didn't get around to making any potions until almost the end of the game, when I got enough holy water from the BF merchants to make some permanent potions.
. Thanks anyway.
. I have a question, the items that vendors sells, after buying them, they do not reappear, right? So, what if buy all the stuff and then run out of potions and food?:D I know, I can brew potions, but what if I run out of herbs? 
