Hello there,
Based on your description what you need is called Digital Products Store.
I have sell some of my work since 2014,
Its built from Core of PHP instead of using ready made CMS like open cart, magento or similar.
The most important thing about selling digital products, especially E-Books is it should have to support Instant Payment Notification (IPN).
Instant Payment Notification allows you to integrate your online payment gateway(example PayPal) with your website's back-end operations, so you get immediate notification and authentication of the payments received. So it works automatically. You can make sales during sleep or during Weekend when you out of work. And no need to verifying your customer payment, because its done by the Paypal and the back-end of your Bookstore.
If i misunderstood your requirements, just ignore this message.
Thank you