The IPN page is not a page that the user will return to.
The IPN Page is a page that the paypal system will make a post to once the transaction is processed to let your site know if the transaction is successful or not.
eCart will not create this page, you will need to create the IPN page.
On the bindings panel, click the plus button and select eCart -> Checkout -> Checkout form response.
from the list of gateways select Paypal IPN Cart.
This will give you bindings to use for the information that paypal will send.
the IP page will need to update the orders table to set the status of the order based on the result sent in form paypal.
see the paypal site for more details on IPN: