Alex replied:
Hi there, I apologize for any inconvenience this has caused.

It looks like your transaction was canceled due to appearing high risk, and if you were charged a refund has been sent to your bank account. All of our risk analysis is performed by our self-learning risk-detection algorithm. Coinbase is constantly under attack by scammers attempting to defraud us of Bitcoin, and as a result we've been forced to implement automated security measures in an effort to protect the community and our site.

I know this must be frustrating to happen, but unfortunately we cannot yet guarantee the completion of every bitcoin order or the verification of every account due to the high risk nature of selling bitcoin online. Thank you for your understanding in this matter.