A preauth on a card reserves the credit without actually completing the transaction. The credit is still there but not available for use. You can complete the

A preauth on a card reserves the credit without actually completing the transaction. The credit is still there but not available for use. You can complete the transaction by sending a 'completion' request, at which point the credit is gone. The preauth also has an expiry. If the preauth expires, the funds are released with no transaction taking place. So when a user signs up, take a preauth. If you have any reporting on your sales, like this guy, you can check for any anomalies before fulfilling orders. Anything that looks odd, don't complete the order.