Not really that risky. If it was a naked call like in markets, then it would be really risky, but in this case you risk very little since there is no contractual obligation to send the actual book if you can't get it. You can reimburse and call it a day.
The calculations would have to consider the probability of not getting the book under the price of sale, the probability of not getting it over price of sale (investing the difference in avoiding a bad rating), and the chance that you won't receive a bad rating just for reimbursing it under whatever excuse. You can also ponder taking a bad rate eventually.
In other markets you'd have to basically get the book at any cost, which would make it really risky.
And in fact, in used book "arbitrage", the seller always has a very, very good excuse, which is that s/he "sold the book already" to someone in s/her imaginary brick-and-mortar store.
I was mostly joking but if you were really going to do this you'd have to develop strategies to reduce the chances of this happening and mitigate it when it does.
Maybe if you have a slightly bad reputation on Amazon that will keep people from buying your book. Maybe you can purchase the book locally and send it to the buyer in a pinch. Maybe you can purchase the book from the actual seller and just use the buyer's shipping address. Etc.
The calculations would have to consider the probability of not getting the book under the price of sale, the probability of not getting it over price of sale (investing the difference in avoiding a bad rating), and the chance that you won't receive a bad rating just for reimbursing it under whatever excuse. You can also ponder taking a bad rate eventually.
In other markets you'd have to basically get the book at any cost, which would make it really risky.