[Plugin: MarketPress – WordPress eCommerce] Bug Report, MarketPress

Registering of Post Type “product” is missing a variable for proper usage with other plugins that also use the archive.

File: marketpress.php
Line: 726 -> Add

'has_archive' => true,

What this will do is set the “is_post_type_archive’ bit in the $query to true.

http://wordpress.org/extend/plugins/wordpress-ecommerce/