Dear Lera,
I suspect that this has something to do with certain emails that were sent in private and to forums that had links to web pages with known Trojan Malware.

~bilbo
N.E. Ohio

There are many, many scam methods in internet. A simple recommendation to protect your Credit Card number is simply, do not provide the number.

Never never provide the Credit card number to an unknown vendor. If you are tempted by the bargain offered and are willing to accept the risk it's OK, but always pay through an intermediating payer such as Paypal.

You may lose your money all the same, but at least the bogus vendor will not have your credit card number. As most scammers will not accept Paypal as paying media, it might hint you to a suspicious vendor (not necessary a scam, but this should raise a yellow flag for you).