In the twenty-eleven theme there is a body_class filter that adds a "singular" class that it uses for some of the styles and those styles seem to be expecting there to be no right sidebar.
Here is code you can use in a child theme (rather than making modifications directly to the function in twenty-eleven). Add this to the child theme's functions.php, to remove the twenty-eleven filter and use a customized version instead, that adds !wpsc_is_single_product() to the if statement:
/**
* Changes the body classes function of twentyeleven to exclude wpsc single product pages from getting singular class
*/
function my_child_body_classes( $classes ) {
if ( function_exists( 'is_multi_author' ) && ! is_multi_author() )
$classes[] = 'single-author';
if ( is_singular() && ! is_home() && ! is_page_template( 'showcase.php' ) && ! is_page_template( 'sidebar-page.php' ) && ! wpsc_is_single_product() )
$classes[] = 'singular';
return $classes;
}
/**
* remove twentyeleven filter and add custom filter
*/
function my_child_change_body_classes() {
remove_filter( 'body_class', 'twentyeleven_body_classes' );
add_filter( 'body_class', 'my_child_body_classes' );
}
add_action( 'init', 'my_child_change_body_classes' );