2 ways I believe:
1) set up an email account that scans messages for key works (ie, from paypal email) and replies with a standard message where to download the ebook
2) write a script that is called by paypal's ipn system, which you can trigger for every sale from the paypal account settings
I think that there is also some sophisticated software that does all this, plus leaves feedback, etc - but don't know what it is called.
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