BF merchants restock each time you enter the BF, but can hang on to things you sell them. During each act, I would sell stuff to the potion dealer there periodically, so as not to clutter up the inventory of the ones that may have useful equipment, etc (or holy water, though that is very rare; if one has one, they all will). At the end of the act I would sell remaining loot to everyone by entering and leaving the BF a few times in a row.