I don't agree with selling them for real money, that's absurd (and I think Nintendo said it's against ToS or something, but IDK if they actually do anything to punish people who do it). But there's not a lot I'm spending IGBs or TBT on (as opposed to real money)... so if someone else has a villager I want that's moving out, and I have extra IGB/TBT, and they need some... why NOT make a deal? I've bought two villagers like that already and am quite happy I did.
But, anyway, I think demand is just down a lot. When this game was brand new so many people were RACING to get the newest, coolest villagers. Now, most of the people who were in a rush... well, they have them. Or they lost interest. Timing plays a big role, too, especially on this website (since it's not as active as some, but I prefer it because the community is so pleasant). I've seen the same villager sell for a wildly different price the night before vs. the next morning. I've even seen people spend a bunch on a villager... only to later see the SAME villager listed as free, only to get voided 'cause no one takes them up on it. A lot of it is just getting lucky with finding the right person at the right time. Especially since you can only move in villagers that way every so often- I think the market would pick up a bit if there was an option to kick villagers out when inviting from someone's island, similar to inviting from the campsite.
That's another thing. Some people seemed to think the economy for villagers was indefinite and would always be a reliable way to make money/NMTs and every villager was worth their weight in NMTs. Not even close. The market was over the top to begin with and it's starting to level out now, and there are some people who refuse to admit that. That makes the market even MORE confusing.