If someone bought one of my books and then, for instance, realized they'd already bought it from another store, that might be the buyer's mistake, but I'd have no problem with the store offering a refund. I don't like it when I see a return appear on my Kindle sales page, but it's still better having a reader walk away annoyed.
I've requested refunds a few times, when books had formatting problems or something that were just too annoying to ignore. Sony, BN (I think), and Fictionwise have all responded to such requests, though it required going through customer service. (In at least one case, though, I was really just asking them to fix the error rather than issue a refund, but that was more than they could handle.)