It doesn't matter what build you have, good gear really counts in all cases.
I've never bought any gear, and even though I had the money for it (at my level anyways o: ) I didn't buy anything so I was pretty weak.
For ThievingSix:
You'll be surprised at how much difference STR makes then. If you're not DEX, and you're an xbow user without too much DEX gear, single-target poisons miss a lot (thank god for DEX gear >w<)