I have this bug too, and I am not using MarketPress. I am using a custom post type that I created.
The bug arises when I add
'hierarchical' => true to the register_post_type arguments. It doesn't matter whether or not I have the post type set to support page_attributes or not, I get the bug either way.
Here's a pastebin link to the relevant code, in case it's helpful.
I'm using WP 3.5.1 and a custom theme. In addition to Simple Page Ordering, I am using the following plug-ins:my custom post type plugin, Akismet, Better Internal Link Search, Broken Link Checker, Contact Form 7, Dynamic Widgets, JetPack, Lazy Load, Shortcode Exec PHP, TinyMCE Email Button, User Switching, WordFence Security, and Yet Another Related Posts
I tried deactivating all my plug-ins except my custom post type and Simple Page Ordering, and this did not fix the problem.
If I can give you any more info to help troubleshotot, just let me know.