I do feel in some cases it can be a bit much, but some players will actively go out of their way to time travel, wait for a rare villager to appear in the campsite, invite them to move in, time travel them in, then, whether using amiibo or more time traveling, get the newly-acquired animal to move out. This process can take an hour or more of real-world time, depending on how efficient you are and methods you use.
If somebody is going through all that work, then why shouldn't they charge? They get compensated for their time, and the person buying gets the animal they want without having to wait weeks or months for the RNG. Seems like a win-win.
Capitalism. Supply and demand (so to speak). People know certain villagers are popular to people, so the owner decides who gets the privilege of having the villager by charging/ auctioning for that privilege. Is it greedy? That's kinda a grey area...that falls into ethics. Is it fair? No. The richest get the villager. On the other hand; who is to say that they wanted the villager more/ less than those with fewer Bells? By no means am I trying to defend this practice. I am trying to explain my thoughts on it though. I hope this helped in some way.
The folloing statement is true; the previous statement is false.
Come visit my dream town!
I'd do give a villager to someone for free!! I'd be more the happy to t-t to move them out simply because I'd want another villager to move in, I get that it takes time for people to do that and they would like some stuff in return but that's their choice.