In addition, many people sell or give away old computers. Using the PSN to identify the user could be a huge security breach. A computer with an upgradable processor would change its PSN each time it was upgraded. Let's not even consider Internet cafes or libraries. The opposite problem is also common.
Many people use more than one computer to transact business.
I have a laptop, a home PC and several workstations in the office. I may use any of them to conduct my business and make my purchases. Any system which required me to do everything from a single CPU would not get my business for long. Cottrell, not surprisingly, also says there is nothing wrong with anonymity. There is no reason that trust must be connected with identity.