I agree that some of the villagers are WAY too expensive, for me even 50 NMT seems too much (but I'm a poor gurl ok), but... the IDEA of selling villagers is completely valid for me. You want to get a dreamie when your villager moves out so you need NMT or some other resources AND, what's more, you trust the person who buys the villager that they won't make some mess on your island.
I, personally, want to sell my villagers who are going to move out, not to charge 100 NMT for one, but to get a chance of getting another dreamie. So I'm fine with someone paying me 10 NMT or less, or some wishlist items or a bit of Bells, or even, if someone really really wants that villager, just "okay, you can just water my flowers and take villager X".
So yeah, I totally agree with saying that overpaying for villagers is wrong, but you said yourself that you hate the idea of selling villagers... and that's wrong for me as well. Because personally I would be just unhappy and frustrated with losing a lot of NMTs on mystery island if I want some villager, if I could just easily buy him for the same or less amount.