When I first started selling products online especially e-books, I used PayPal to accept payments. 6 to 7 years ago PayPal was a great way to accept payments online for e-books because you didn't have to worry about setting up a merchant account. But there are a few major issues with excepting payments using PayPal now that weren't really problems back then.
One of the first issues that you'll find when excepting payments with PayPal is the fact that buyers take advantage of the system that PayPal uses. The system was set up to protect consumers but there are a few consumers out there that take advantage of this system to get products that they want without having to pay for them. What they do is they buy the product, then they put in a dispute with PayPal stating that the product wasn't as described. The issue with this is that it's really hard to prove that your product was actually as described. So if you're willing to live with the fact that they are a small percentage of people out there that will take advantage of this loophole in the system you won't really have that big of a problem with it.
The other reason that you may not want to use PayPal to accept payments when selling e-books is the fact that a majority of your customers may not even know how to use PayPal. You have to make sure you know who your target customer is and find out whether or not they're even familiar with PayPal. I sell a lot of e-books that teach people how to start their own Internet business, so most of those people do have PayPal accounts. But since they do have accounts with them, they also know how to take advantage of PayPal's system. So it will always be a double-edged sword to use PayPal but like that old saying goes, you can't live with them and you can't live without them.
In the end it doesn't really hurt to use PayPal if you have another payment method such as your own merchant accounts as your primary means of excepting payments.