in a configuration that is useful transactions via paypal
whether the necessary configuration for config.sandbox.inc.php:

define ('SANDBOX_SELLER_EMAIL', '');
define ('SANDBOX_BUYER_EMAIL', '');

leave empty or needs to be set?

or change
<Input type = "hidden" name = "notify_url" value = "http://www.domain.com/web/ipn/ipn.php">

to
<Input type = "hidden" name = "notify_url" value = "http: //www.domain.com/web/product / ...">

or

<Input type = "hidden" name = "return" value = "http: //www.domain.com/product / ....">

<Input type = "hidden" name = "cancel_return" value = "http://www.domain.com/cancel_buy.php">

>leave empty or needs to be set?

just leave it empty. It's for those who are testing it under PayPal sandbox.