What do you think if I refuse to refund a textbook I sold on kijiji?
That sounds greedy. Why not just refund the book?
I've had a bad experience with a customer a couple of years ago, so now I can refuse to refund any textbook.
That seems a little scammy to me. You should try to find a way to make both you and the customer happy, so that they do not leave a bad review.
People are already so stupid because they buy used textbook then give me bad reviews because it's not like new.
There will always be bad experiences when selling online. You are wise enough to know that not all customers are created equal, so maybe doing the right thing by giving refunds is more admirable. Plus, it might bring in more business for you if people see that you are very kind and flexible.