During the first half of this year, 23 million credit cards were stolen worldwide, according to cyber threat intelligence company Sixgill. About two-thirds of those stolen card numbers were issued in the U. For many, gaining credit card numbers is about more than simply making fraudulent purchases — although they do that as well, cyber-security expert Joseph Steinberg tells CNBC Make It. Credit card numbers can be converted into cash by buying up gift cards and purchasing easily sellable items to resell through online marketplaces such as eBay, Steinberg says. Then there are the criminals who are interested in the big hauls. In many instances, the fraudster is actually selling your credit card number to other cyber criminals. Let’s say you have a trove of credit card data, such as the Marriott data breach which compromised, among other pieces of data, credit and debit card payment information for million people.