A method of and system for making purchases over a computer network using an ATM card or the like is provided. In accordance with the invention, a consumer transmits his ATM card number over the network to an on-line merchant. The on-line merchant then forwards the ATM card number to a third party contractor, such as a bank, that will oversee and authorize the transaction. Simultaneously or thereafter, the consumer transmits his PIN over the network to the third party contractor, who verifies that the ATM card number and PIN are valid.