I have not purchased anything except potions. Everything else I get from drops and treasures. Detect hidden ftw.
I make most of my potions. There are a few I don't have yet, and need to buy, but otherwise, most of the cash I spend is on lockpicks (my lockpick skill is like, 3) and repairs.
Oh, and sometimes it's easier to buy reagants/shards than wait for them to drop/hunt.