At the last moment after confirming the order I get the message: "Whoops. Looks like you've missed a step".
I'm able to replicate it in a live dummy server I'm running. If you want to check it just let me know.
I saw the thread of your site: http://premium.wpmudev.org/forums/topic/php-session-info-and-marketpress-whoops-looks-like-youve-missed-a-step-message#post-118373
Obviously I'm not registered yet (but willing to do it)so I can't post there.
A developer is suggesting that the theme is reloading the page. (I just check the address "Redirect Checker": http://www.internetofficer.com/seo-tool/redirect-check/ and looks fine.
Hope this helps.