You must use an Authenticator to use RMAH in D3! : Diablo 3 General Discussions

http://eu.battle.net/d3/en/forum/topic/4551610157#1

In preparation for the upcoming launch of the real-money auction house, we will be making some key changes related to the account-security measures required to use Battle.net Balance. Later today, the following updates will be made to the Terms of Use available on our website, and when the real-money auction house goes live, all players will be asked to re-read and accept these changes the next time they play Diablo III.

Battle.net Balance and Authenticator Update
With the introduction of the real-money auction house, account security will become more important than ever. To help ensure that players have a positive experience when using the real-money auction house, we’ve made some adjustments to how players can use and access their Battle.net Balance.

Starting today, in order to add to your Battle.net Balance, players will be required to have a Battle.net Authenticator or Battle.net Mobile Authenticator attached to their Battle.net account. For clarity, this means you’ll need to have an Authenticator to add to your balance via Battle.net Account Management or to send the proceeds of your real-money auction house sales to your Battle.net Balance.

Please note that players who previously added Battle.net Balance to their account prior to this change will be able to use it to make eligible purchases on Battle.net and in the auction house without attaching an Authenticator. However, an Authenticator will be required to add to your balance in the future, as explained above.

While we understand that this creates an extra step for players during the login process, we believe this added layer of account protection will help foster a safer auction house environment for all of our players.

You can learn more about the Battle.net Authenticator, Battle.net Mobile Authenticator, and other account security information by clicking here.