There are a two ways I recall. One requires someone with alchemy skill (of fairly high level for smoe potions/mixtures). You can merge potions/powders to make better ones. The "better ones" are usually (maybe always) worth more much more than the cost of the ingredients. Like Lt. Heal + Mod. Heal = Hv. heal .... there are quite a few posts on the topic of money making and alchemists.Another, much more laborious process is to split every bundle of arrows and rocks you have into single quantities (i.e. 1 arrow). Often a bundle of 5-10 of these items will sell for 1 gold piece, but since the game doesn't have 1/2 gold pieces, etc. So, instead of selling 10 arrows for 1 gp, you sell one arrow for one gold piece, but you do it ten time.
I think the percent return is higher on the second method, though takes much longer.
Tools for Wizardry(r) 7