Maomao has expressed multiple times that she doesn't want to be sold for sex work by the old hag. Obviously because it would interfere with her experiments and research.
I suspect that the old hag would never actually sell Maomao for sex work. Maomao's skillset is far too valuable. The three princesses would also throw a fit. The five of them are a very tightly knit family unit even within Verdigris House.
Maomao grumbles about the old hag all the time, but Verdigris House has a significant aftercare facility for girls who succumb to one illness or another. Luomen presumably does a lot of the work, but we've already seen Maomao doing it as well.
The old hag is has been shown to be lot more caring than Maomao's statements give her credit for.