Word press is free to use to use, easy to install and flexible enough to do any customization and fun to work on. I think these are the reasons due to which word press is gaining popularity for ecommerce based site.
A Few Easy steps to construct your standard ecommerce site
full tutorial at : PSD to Wordpress Tutorial
https://www.squidoo.com/construct-a-standard-e-commerce-site-on-your-own-using-wordpres