The security issues in e-commerce are resolved by the setting up of the Public Key Infrastructure (PKI) supported by a legal framework through legislations on its effect and meaning.
With regard to the so-called brick and mortal traditional commerce, transactions are paper-based. Papers such as quotation, purchase order, contracts, invoices, cheques and receipts are used. The papers serve evidential and communication purposes between the parties. As to security measures adopted in traditional commerce, it is done by use of signature and countersignature. The persons signing the documents are identified to be the parties involved in the transaction.
However, electronic transactions are carried out differently. If you ordered a book online through a web-site, orders and receipts are made electronically and through electronic communication such as emails. To authenticate a person's identity, web-sites do so by supply of personal data, credit card data and registration. Registration will give you a user account and password enabling you to be identified by the web-site who you are. However, the authentication is not done by a means enjoying trust, it is not a reasonably reliable way to authenticate a person's identity. That is why digital signature and certification authority are promoted.