Hi Gary, the plugin has been tested to work on all major internet browsers (eg. IE, Firefox, Chrome, Safari). I agree that it would be idiotic to tell the customer that they can only buy products using a specific browser.
With that said, I just tested the link you gave me in IE8 from my computer and it seemed to work fine. The browser settings of IE8 on my computer is set to default (this is what most people will be using).
I have a suspicion that something is going wrong with the browser settings in IE8 on your PC (after all it’s IE 
or your server.
Lets try to pinpoint the issue… I know that my site uses the WP eStore plugin and it works on all major browsers. So can you please go to the following link and test the cart out? (don’t forget to do this in IE)
If it doesn’t work then I would say the IE8 settings on your PC maybe acting up (Please try setting it to the default then slowly apply your changes)
Let me know what you see.