Do I understand correctly that when you say...
"Checkouts paid through PayPal, Google Checkout, ClickBank, 2CheckOut, or TrialPay would occur on those payment processors' respective checkout sites, so we have no control over them, but PayPal checkouts in particular do not have any opt-in/-out checkbox, so all buyers who pay by PayPal would be opted-in automatically."
...this means if I use Paypal (not Pro) as my cart, then the person making the purchase is never brought to that screen where the checkbox appears. But they are simply added automatically to the Buyers List which is simply something that allows us to have the ability to send fresh download links, as an example, to those who have previously purchased from us. Is this correct?