The whole site is currently down at the moment, claiming that they have not been hacked.

We are currently experiencing a hardware issue which is causing all AbeBooks sites and services to be unavailable. Buyer and seller information remains secure and is not at risk.

We are working to resolve the issue as quickly as possible. We do not yet have an estimated time when this is going to be resolved.

Customer Support tools are also affected and they are unable to respond to inquiries at this time so please check our Facebook and Twitter accounts for updates.

https://twitter.com/abebooks
https://www.facebook.com/abebooks/

Customer Support will resume responding to all inquiries as soon as they are able. We apologize for the inconvenience and interruption of services.

Sincerely,
The AbeBooks Team

As part of Amazon, I would expect the site to be running on Amazon Web Services, which is meant to stop this sort of scenario happening. Also, it appears that exactly the same issue happened last September and the site was down for 2 days then.