I received a refund

I feel guilty, I received a refund for an online purchase as well as my stuff!!

I don't know what to do, about 20 days I purchased a book at abebooks.uk. The estimated delivery date was about October 7. However, it was already Oct 11 and I had no news about it, so I decided to request a refund. Today I received my stuff early, as well as the refund I had asked to be issued 2 days ago. What should I do ? Should I tell the seller that I received my book after all, and give him the money back (de-refund)? Please, tell me what to do, I know it's unethical to do so.

Though, I'm also aware it's not completely my fault, and it's kind of my right to be refunded. Like the pizzas slogan "15 minutes or it0s free2. Plus those companies do not lose money either, do they ? I guess it's not such a big deal for them. Bu yet, I feel guilty, because Abebooks is a site for independent seller, and this kind of things do affect them and make them lose money. Do you think it's so bad to keep both, the money and the book ? I feel terribly guilty.