The Google Authenticator plugin for Sell&Sell gives you two-factor authentication using the Google Authenticator app for Android/iPhone/Blackberry.
If you are security aware, you may already have the Google Authenticator app installed on your smartphone, using it for two-factor authentication.
The two-factor authentication requirement can be enabled on a per-user basis. You could enable it for your administrator account, but log in as usual with less privileged accounts.