Before I say anything else, this is a bad adaptation of the source--just as everyone else who's commented on it has said, for the same reasons.
That said...I see the marks. I get that she's traumatized. What
@Speak_Evil just said also makes a bit of sense: intense fear can override and suppress all logic, especially in someone who's physiologically under greater influence of the chemical processes that govern and evoke emotion (i.e., women). But though she was already hoping to be saved by Eleanor, she almost immediately recognized Egil as Eleanor's master--whom did she think was going to throw the money around, when Eleanor was a slave herself, not long ago? And when it was all said and done, and she stepped out of the auction hall, she was overcome with relief--why? Obviously she felt like she was saved--how would leaving safety for the unknown be a good idea?
Perhaps Selina's PTSD took hold shortly after Eleanor declined her offer to run away with her, since she had time to think more about Egil--another potential threat, to her traumatized mind--and less about Eleanor, but no time at all to think about the two of them
together, and why Eleanor looked so good for a slave, especially compared to Selina herself.
The author really should have gone with the source's story. Not like that noble couldn't/wouldn't have tried to play himself out of position--they love doing that, anyway.