This is a shortcode example for enforcing a limit on the number of times users can buy posts that has been set for sale via the myCRED Sell Content add-on.

WP Shortcode: myCRED Sell Content Limit

/**
 * Custom Shortcode: Limited Content Purchases
 * Example WordPress shortcode to impose a maximum number of times
 * a user can purchase a post in WordPress using the myCRED Sell Content add-on.
 * Requires myCRED 1.4 or higher!
 * @version 1.1
 */
add_shortcode( 'mycred_sell_this_limited', 'mycred_render_sell_this_limited' );
function mycred_render_sell_this_limited( $atts, $content = NULL ) {
	// First make sure we do not get an error if myCRED gets deactivated
	if ( ! function_exists( 'mycred' ) ) return;

	// If we are logged in, check first if the current user has
	// purchases left.
	if ( is_user_logged_in() ) {
		global $wpdb;

		// Load myCRED
		$mycred = mycred();

		// Lets query the database
		$count = $wpdb->get_var( $wpdb->prepare( "
			SELECT COUNT(*) 
			FROM {$mycred->log_table} 
			WHERE ref = %s 
			AND user_id = %d 
			AND ref_id = %d;", 'buy_content', get_current_user_id(), $GLOBALS['post']->ID ) );

		// If user reached the maximum allowed purchased, return something else
		// instead of the shortcode. This will prevent them from being able to make further
		// purchases.

		// In this example, we return a simple message. You can change this to anything
		// as long as it gets returned.
		if ( $count >= 3 )
			return 'I am sorry but you can only buy this post max 3 times.';
	}

	// Pass on any attributes we might be using
	$attr = '';
	if ( is_array( $atts ) && ! empty( $atts ) ) {
		foreach ( $atts as $key => $value )
			$attr .= $key . '="' . $value . '" ';
	}

	// Lets run the original myCRED shortcode
	return do_shortcode( '[mycred_sell_this ' . $attr . ']' . $content . '[/mycred_sell_this]' );
}