Basically, software running on your website listens for these events and sets access flags so that customers can download the book after payment (instantly)

If you want to write the software yourself, here’s a toolkit I make available:
(Override the methods to verify TXN’s, amounts, etc..)

If you don’t wan to write your own software, there’s PayVeX:
(Full disclosure, I own PayVeX)

PayVeX works with paypal AND pecunix, which may help you sell e-books to international customers who don’t have paypal accounts, but it’s optional.

As far as preventing people from sharing, this is tricky! there are password protected PDF’s and so-on, however, they cause problems, your readers won’t be very happy with you.

PayVeX takes some measures to prevent people from sharing download links, but if they have the file.. they can easily forward it in email as an attachment.

Another approach would be to watermark each PDF, this is sort of overkill and doesn’t really prevent anyone from distributing it.

If people want to, they’ll always find a way to bypass copy protection schemes, in some cases, it could even have an inverse effect – people either won’t buy your e-book because it’s “DRM protected” or they’ll take it as a challenge to find a way to copy it. (the forbidden fruit theory)

Personally, I think your best bet is to prevent them from wanting to in the first place, interact with your readers as much as possible, so they know you as a real live person.

People kick and punch vending machines with impunity, but they’re still a little reluctant to kick and punch people who provide them with things they like.

I know that sounds overly simplistic…