I recently installed Marketpress Pro (the problem was the same under the Lite version, though) on my site and it works well...
Except that when I click "Buy Now", I'm taking to my shopping cart which is blank. All I see is my header and my footer with a big blank space in between.
I didn't think "Buy Now" was supposed to take you to a shopping cart but rather right to checkout, but that's another story and not a big deal.
I did see one poster say he disabled a permalink plug-in which caused this problem, but I don't think I have any such plug-ins.
Here's the test item that shows what I'm asking about: