Some of your widgets don't drag and drop correctly in IE. The error that you get is:
Message: 'this.helper.0.style' is null or not an object
After a bunch of hassles trying to figure it out, I found that it's because you are missing a couple closing tags from labels. Here's what I changed:
* In /marketpress-includes/marketpress-ms.php I added a close label tag for the 'Content To Show:' label.
* In marketpress.php I added a closing label tag for the 'Order Products By:' label.