[muyuu]

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.

[morphogenesis]

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.