I have in mind an automated system where people would buy ebooks via PayPal. After a purchase, a download (prefferably unique) link would appear. Is there any such free software already available on the market?
You need a programmer, if you are not one yourself. The system you desire is really simple. I would suggest you come up with another idea, however. I'm not familiar with ClickJunction, but isn't that what they do? Plus, getting the rights to e-books would be expensive. I don't see this as being a worthwhile venture...
No, I'm looking for something FREE. I don't have any money to spend right now, perhaps in the future. Thank you for your reply.
Then download some free PHP/ ASP/ .Net books/ websites and learn how to program it yourself? As Zac says, it isnt overly complex to do