Paypal's developer site has a number of possible
third-party vendors to select from. (You may need to become a member to access this.)
My site uses Tools4Webmasters' t4w-d-goods, a perl script, but I'm not sure whether Tools4Webmasters still offers this app. Once you put your e-books into a folder, file-encoded for security, the t4w-d-goods sw will interface with the Paypal purchase process and send the recipient the appropriate e-book(s) automatically. That prevents someone from simply downloading the e-books at-will, and allows you to go about your life while your e-books sell themselves.
If t4w-d-goods is no longer available, other scripts will do the same job.