Trading Chars

First off I have to say Blizzard does not support you to do this.

But to trade characters you would have to find someone else you can trust or use a third party place like:
http://www.accounts.net
http://www.buymmoaccounts.com

Some of these places allow you to trade in your account for on-site credit that you can use toward another account.

If you don't use a third-party to buffer the transaction then it can be very risky. At a minimum I would talk to the person on the phone that you are exchanging the accounts with and get some information from them. Where they live, etc. Get the account security question, their date of birth, and even the last 4 of the credit card used on the account; and don't forget, the account username/password.