I personally think that selling villagers is fine.
It's like my mom always says when I tell her I need to do something in a game- "No, it's just pixels. If your pixel horse is dying, you can just make another horse out of pixels."
I mean, yeah, you can't just summon villagers, but hey, it's not like we're selling slaves. IRL, would you get freaked out if someone sold a dog at a pet store? No, because that dog is going to a loving home with people who want them. I might not want my villager, but somebody else probably would love that villager.
Several people have said that bells are easy to make, so if bells are easy to make, what's wrong with people buying things with easy-to-make bells that they can easily get again? Maybe I don't like Goose, but chances are, somebody who likes Goose will want him and will be willing to pay. I think it's only fair that I make a little profit from all the countless resets I've done to achieve Goose. Now, Goose I'd probably give away for free, but I'm just using him as an example.
If I were to find Marshal and two people both asked for him, one providing nothing to compensate me for the time I spent getting him, and one willing to pay a bunch of bell bags, I'm obviously going to want to profit from Marshal. This is especially seen in the cuter villagers who people have a hard time letting go of, because people get attatched to them and want some money so they can find something just as good to make up for the time they spent getting that villager and then having to get rid of them just when they thought, "Hm. Maybe I like this villager."
Anyways, when people sell villagers, we're not selling slaves. We're not selling people, we're not saying, "Oh, I don't like this villager, so let me turn a profit and get rid of them." We're giving them to people who want that villager, and IRL, when you want a cute, furry animal, chances are, you pay for that cute, furry animal. That's all the Animal Crossing villagers are; cute, furry animals, which are fun to collect. They aren't real, they're pixels that are fun to look at but eventually we'll all put down the game and move on in life.
Lastly, when people give you bells, it's not like you're giving them actual money. That's just a little bag that somebody drew up and stuck into a video game. All those 99,000 bell bags sitting in your pockets are actually just pixels. Sure, you spent a few hours getting it, but somebody probably spent a few hours getting what you're buying them with. Thus, it's a fair trade.
Now, this is just my opinion. Other people have their own opinions. But this is mine, and you can agree or disagree; I don't really care, because, in the end, we're just arguing over which is better: selling pixel animals for pixel money, or just giving them away. None of it's real, so why does it matter what people choose to do with their villagers? I usually give them away, but occasionally, I'm going to want a bit of profit. Why? Because I can't build things like the campsite and PWPs that increase the worth of my town and make animals want to move in without that profit.
Anyways, that post dragged out a bit. But whateves.