Responsive Web Design (RWD) is an approach to web design aimed at allowing desktop web pages to be viewed in response to the size of the screen or web browser one is viewing with. Responsive Web Design makes your web page look good on all devices (desktops, tablets, and phones). Responsive Web Design is about using CSS and HTML to resize, hide, shrink, enlarge, or move the content to make it look good on any screen.

Not only the e-commerce website, but all other websites also use responsive web designing. So, I can not recommend one single web site.