Plot hole: There is no way Anna could know that Hester was going to be at the auction. The circumstances that led to Hester and Tom being captured can't possibly have spread to the Anti-Traction League, they were immediately taken to the slave auction after being captured. Anna also instantly recognizes Hester when she sees her despite the fact that Hester was 8 years old the last time she would have seen her.
Suggested correction: Anna might have very well just been there to do anti-tractionist things, it was an auction made of towns after all a prime target to hit many options at once or hide bombs on for a bigger city to gobble up and go off later. Once she saw Hester her plans simply changed. As for how she recognized her, it is possible Anna had been tracking / keeping an eye on her. It wouldn't be impossible to ask around for a scar faced girl moving from town to town and might have gotten a pic somewhere.
These are all wild guesses that aren't confirmed by the film itself. They do not qualify as corrections.
