I run an ebook store myself. Use a legal disclaimer and also include in the terms of services page a statement to the effect that due to the product being delivered digitally refunds will be processed on a case by case basis...and we reserve the right to refuse refunds.
I also had this issue previously when we first launched. Now if they open a dispute within PayPal all I do is refer to the terms of services pages ensuring to paste the direct link in my reply, and the customer and paypal are pretty quick to close the dispute.
On the other side of the coin, if an issue is found with one of our products or the claim is valid, then we prefer to offer them a free product of their choice which would be slightly more expensive then the one purchased. Just another way to keep that customer happy and coming back to purchase more products from us.