add_filter( 'wcfm_sell_items_catalog_args', function( $args ) {
    if( isset( $args['author__not_in'] ) ) {
        $vendors = wp_list_pluck( get_users(
            array( 'role__in' => array( 'wcfm_vendor' ) )
        ), 'ID' );

        if( !empty( $vendors ) ) $args['author__not_in'] = $vendors;
    }
    return $args;
} );