Creating a shortcode to accept multiple IDs

I am developing a shortcode to display a list of products in the accessories category. It works but I am trying to set it up so I can enter the id numbers like [my_wpsc_access post__in=”1,2,3,4″]

add_shortcode('my_wpsc_access', 'my_wpsc_access_shortcode');
function my_wpsc_access_shortcode($atts) 

{

global $post;

extract(shortcode_atts( array(

	'post_type' => 'wpsc-product',
	'posts_per_page' => -1,
	'wpsc_product_category'=>'accessories',
	'post__in' => array(1,3,),
	), $atts ) );

	$args = array(
 	'post_type' => $post_type,
	'posts_per_page'  => $posts_per_page,
	'wpsc_product_category'=>$accessories,
	'post__in' => $post_in);

$myquery = new WP_Query($args);

That does not work. But if I do this it works.

$myquery = new WP_Query( array( 'post_type' => 'wpsc-product', 'posts_per_page' => 3, 'wpsc_product_category'=>'accessories', 'post__in' => array(1101,1730,1191,)) );