It's a bit naive to think that just because you pay, the site is going to be more secure. That hasn't proven true of any software in history. This could have happened to any site, pay or no pay. They can catch most of the stuff most of time, but no one can catch everything all of the time. In the 2 years I've been using Auctiva, this hasn't happened until yesterday. But I remember Ebay being attacked, at least once. BIG services can be compromised--little services can be compromised. For-pay and free services can be compromised. Never for a moment think that anything you use over the net is completely safe.
Yes, it's terrible, and probably MOST of us have lost potential customers due to this attack, but there's absolutely no reason to consider it anything other than one of the unavoidable risks of working over the internet. If you had a physical store, it would be at risk of falling prey to theft, vandalism, natural disaster, people crashing their cars into the building, and other various problems. These things can happen, and every once in a while, they do.