Although we may end up becoming attached to it, we have no money so we'll just have to bear it should that happen.
Since a slave is another person, having to sell them would be very sad. If for some reason I need to sell a slave whom I've become close with like Raphtalia, I'm not sure I would be able to do it.
However, monsters aren't able to talk, so no matter what it'll just be slightly painful.
Leaving them behind with a 'I hope you meet a good master'.
"So you also provide those services?"[8]
"I'm excited by how insightful Hero-sama is! Yes!