MarketPress is the best free eCommerce plugin available for WordPress. Beautiful, powerful and easy to use.
For store functionality, it works. But it's a pain and missing some very obvious features.
Coupon codes work across the whole store, not just for a particular product.
It doesn't easily allow for feedback from customers on custom products without getting very cumbersome.
I used this version then bought the full version.
Recommendation: Try other WP store plugins first.