You can get gold by looking everything, using the resurrect trick to repeatedly kill a strong opponent that always drops something, quicksave and reload for chests, etc, to make sure you get better quality loot/equipment, repair equipment before selling....
The game tends to get easier as it progresses, so you will not likely need a full compliment of gold charms. Also, a hotkey for
restoration potions should take care of your mana reserve issues.
The charms I bought were just checking several merchants when dropping off loot, selling stuff, etc. If I was selling stuff and had the patience I sometimes reloaded to try to get spellbooks or charms, but even then narrowing it down to specific charm types would have dramatically cut back on the numbers.
The cellar's hatch
is beside and partly under one of the beds (
screenshot) in the alchemist's house, south west of the Verdistis park.
