I think that the reason sometimes that "unpopular/low tier" villagers go for so much is because people are much less likely to post them than popular villagers. I know that this is pretty true for me. When I had villagers move out of my town who weren't highly in demand, I usually just sent them to the void. But when I was trying to trade for Sherb before I found him on my own, I put in my post that I had Katt and someone DM'ed me desperate to buy her when I thought no one in their right mind would ever want her in their town. I ended up just giving Katt to this person because it was nice to know she was someone's favorite. So sometimes when someone is really desperate for their favorite villager who isn't popular/in demand, they're willing to pay alot for them when they are available since it is sort of hard to find more obscure villagers.