Respond.js must be included after all css files. eShop includes the css file named eshop.css after the files included by header.php in the theme. If the respond.js is included by the header it will be too early to work for styles in the eshop.css file.
The simplest workaround is move the eshop styles into a file that is included by the theme. These styles must be removed from eshop.css so as to not override those included earlier. I've tested this and it works on IE7 and IE8.
It may be sufficient to just move the media queries, but I haven't tested that.