I work exclusively in Keller and in the west Alliance growth project. In this area we do not see many builders unloading at auction because they are offering rock bottom prices to move in to their homes with associated financing. I do not believe it is necessary for you to have an agent at auction. The builder will prefer you to use their contract and as long as you read it carefully and have performed your due diligence on the internet, you can purchase that home on your own. As Bruce mentioned, if there are multiple bidders on a home, the auction price could be more than another builder would sell for in a one on one transaction.
I do receive email from builders in this quickly growing area so if you are interested in working with me , I will represent you until you find exactly what you are looking for!
I also receive email and postings on this site will confirm that not using a Realtor could cost you in hindsight.
Best of luck!