Hey, I noticed something: If I sell a unique girl, they remain in my Inventory window. For instance, I sold 3 unique girls in my most recent game, but when I look at the "Manage Inventory" or "Shop" screens, they're still in the list of girls.
EDIT:
Somehow, they wound up at the Centre. I know I sold them (I sold them from the Dungeon screen), and that they were gone. I don't know how they got to the Centre. Somehow, I got the money from the sale, and autotransfered the girls to the Centre. Is that supposed to happen? Also, this doesn't work with the randoms, just the uniques.