Jérôme Barberin selling Remy to Human Traffickers is fucked up in the first place, but you also have to ask what kind of people would even be interested in buying children in the first place.
So, Gaspard adopts children so he can train them to steal money for him, using their small size and cute appearances to their advantages. What happens when a child ages out? In the novel, he actually murdered a child and was sent to jail for it.
Gaspard being sent to prison in the end. Prison inmates don't look too kindly on child abusers, and they're often at the bottom of the caste system. Consider the fact that Gaspard has multiple cases of child abuse on his record, and there's a chance he won't be in prison for long.
It is not just Gaspard's abuse Mattia has to worry about when stealing, but also the possibility that his victim notices what he's doing. Remember this is 17th-something Century France, there are no social services - nothing is going to stop a civilian from beating the would-be thief into the ground.
The village of Chevanon is very tight-knit and Remy was known by the local community. Now that Jérôme sold her off, wouldn't the townspeople notice and ask where she is? The Barberin's reputation would be ostracized if word got out that he sold his own daughter.
This is lampshaded in the first episode when the townspeople are seen laughing and praising Remy when Jérôme frowns and doesn't understand who that strange child is.
