Once you have successfully registered as an auctioneer, you will see an option called "eMail Bidders" under the My Account -> Members Area -> Selling -> Seller tools.

This is fairly straight forward. There is a drop down menu which offers five groupings of people you might want to contact with the same message. Select the group you wish to contact, fill in the subject line, enter your email message to the group and click on the proceed button to send out an email blast to the group.