Visa and MasterCard require each merchant to perform AVS (Address Verification Service) on all transactions where the card is not present. However, this is just one form of fraud prevention, and there is no guarantee that the transaction is not fraudulent. The merchant is responsible for all transactions accepted, even if AVS is approved. If you feel a transaction is possibly fraudulent, you can contact the cardholder, as well as the Financial Institution that the card is affiliated with, to verify authenticity.