When google created their android market they had a 24 hour refund policy in place, meaning any app you bought could be returned within that window for a refund if you "disliked" the app, this window was later shortened to 15 minutes amidst developer lamentation. The window is probably gone now.
The idea could be improved upon of course. Instead of an artificial real time window, the time you are given to return a digital file could be based on the length of time you actually use the file. For books this is simple, length of book divided by some average reading rate, if you read the book for longer than that, monies are deducted from your account and sent to the creator.