[Armin does plan on keeping it, but he doesn't say so. He addresses Saelfen's concern, instead.]

With all due respect, I have to disagree. After all, this isn't your usual market at all. People here deal in dreams, memories - items that you know, like that mirror. It has value beyond money-- Ah. Well, that can be both a positive and a negative thing.

Point being, if you've been given that item, and the way home is through buying and selling, the only logical conclusion is that selling that mirror would get you a step closer.