WordPress at a glance
action is not described

valid-paypal-standard-ipn-request action-hook . WC 1.0

Usage

add_action( 'valid-paypal-standard-ipn-request', 'action_function_name_6740' );
function action_function_name_6740( $posted ){
	// action...
}
$posted
-

Where the hook is called

WC_Gateway_Paypal_IPN_Handler::check_response()
valid-paypal-standard-ipn-request
woocommerce/includes/gateways/paypal/includes/class-wc-gateway-paypal-ipn-handler.php 49
do_action( 'valid-paypal-standard-ipn-request', $posted );

Where in WP core the hook is used WooCommerce

woocommerce/includes/gateways/paypal/includes/class-wc-gateway-paypal-ipn-handler.php 35
add_action( 'valid-paypal-standard-ipn-request', array( $this, 'valid_response' ) );