I started with around 125 pieces from my own collection for Market Street, and around 75 pieces for Cafe Corner. The pieces I already had were mostly smaller common pieces. Afterwards I placed an order with LEGO Bricks & Pieces to get anything that was overpriced on BrickLink vs. through LEGO directly. I was able to save money on some of the still available but rarer pieces--dark blue bricks in both sets including the arches (thanks to Winter Village Market), dark bluish grey parrots, dark red bricks including the slopes (thanks to Pet Shop), etc.
So far I have stuck with mostly new parts or good used quality. It's been a long road and I'm definitely excited to be finishing up Cafe Corner, but it's been worth the patience in order to keep costs down along the way.