This is a user to user forum with some tmobile moderation that don't have account access so you need to contact Tmobile support using messenger on one of tmobile's social media platforms to like Facebook or Twitter and they can answer this.

Did you ever end up figuring out how to sign up for the tmobile auctions 

you email them at the link provided on a post in here

Hey there! It is really cool that you are interested in participating in the T-Mobile Auction. If you still need help applying to become a bidder, you can message the team right here on the Community. Just send a message to @T-Mobile Auctions and the fantastic team that runs that program will be able to help you out. 

Thank you if you contacted our @T-Mobile Auctions account.  
After doing some additional digging, I found some cool information about a larger auction opportunity if you are interested in bidding on used phones. This program has open registration, and you can easily check out the inventory available. T-Mobile sells a mix of manufacturer and carrier brands. 

Register now at Superior Wireless Auctions | Official Superior Wireless Auction Marketplace (bstock.com)