One person taking a book means others can not see the same book. Acceptable DoS because the owner thinks you may purchase it.
One person taking several books means others can not see the same books. Questionable DoS because the owner thinks you're screwing around.
Many people doing taking several books out in the store leaves the owner to suspect a DDoS attack because there's not usually that many people and none seem to want to buy. It's especially malicious if there's intent.
Ugly thing about DDoS is that since there's unexceptionally large amount of people and no books to check out, loyal and new customers will be put off and may not return causing future loss.
One person taking a book means others can not see the same book. Acceptable DoS because the owner thinks you may purchase it.
One person taking several books means others can not see the same books. Questionable DoS because the owner thinks you're screwing around.
Many people doing taking several books out in the store leaves the owner to suspect a DDoS attack because there's not usually that many people and none seem to want to buy. It's especially malicious if there's intent.
Ugly thing about DDoS is that since there's unexceptionally large amount of people and no books to check out, loyal and new customers will be put off and may not return causing future loss.