I'd say this in order of my purchasing decision thought process:
1) Clarity of images. Product images need to be clear concise and offer views of all angles.
2) Product descriptions and measurements. I believe people read them. It doesn't need to be verbose, but if you're buying clothing, fabrication is important -- and sometimes, you just can't tell from an image.
3) Reviews if relevant. We'd like to know how an item fits on a real person, and if they explain how it fits and they give us their measurements
4) Sizing: Netaporter does this really well by measuring the garment's actual dimensions too and providing a size chart.