It´s not that. I GET the idea of training and selling slaves. I played Jack O nine tails and I LOVED that. The problem is that GD shoot him/herself on the foot asigning Unique Quests to the story slaves AND then having some of those quests as WIP, not implemented yet.Yeah unfortunately a substantial subsection of people playing it are harem builders, who just want to collect as many slaves as possible.
So for example I CAN´T sell Juno because I lose future access to the kingdom of David or i can´t sell Ameril because I lose access to a future raid (maybe profitable) quest. And sure, I can sell michelle after finishing her quest, but then the Reward of the quest (Adrienne) will be devastated.
For now I think the only ones kinda "safe" to sell after the quests are Lovisa (and even she is risky, because GD may want to implement her sister later) and Amelia. Sure you can sell Rebecca, too. but WHO would sell the most profitable slave ever?. Everyone else still has pending quests or WIP quests and since they are unrecoverable, you need to hoard them.
The solution was to either implement a way to recover the slaves after a sell OR just pokemon them, which is the way GD decided to go.
PS: Rebecca-chu I Choose you