The plugin attempts to create thumbnails when product has no attached images. Patch follows.
--- wp-content/plugins/wp-e-commerce/wpsc-includes/product-template.php (tag 3.8.7.6.1)
+++ wp-content/plugins/wp-e-commerce/wpsc-includes/product-template.php (working copy)
@@ -1203,7 +1203,7 @@
// calculate the height based on the ratio of the original demensions
// blame Cameron if this is buggy :P
- if ( $height == 0 || $width ==0 ){
+ if ( isset($thumbnail_id) && ($height == 0 || $width ==0 )){
$attachment_meta = get_post_meta( $thumbnail_id,'_wp_attachment_metadata', false );
$original_width = $attachment_meta[0]['width'];
$original_height = $attachment_meta[0]['height'];