I've never bought or auctioned on villagers so it personally doesn't matter much to me.
I can understand why people would care about it, though. It's like starting with villagers from scratch. In some cases, a villager's clothes are a part of their design and sometimes people like their houses (I think Julian is one of the villagers people want in an original state the most because of his furniture).
You can reset catchphrases but changing their clothes and furniture is trickier. When you send them clothes there's a chance that they won't wear them and will just display them in their houses. Furniture is even worse because sometimes the layout is random. Unless they ask you for something to replace a specific item, they'll just swap out something of the same size so sometimes it ends up looking weird. There are also items you can't buy when you visit them like fish and bug tanks. For both furniture and clothes, you need to be able to find and buy them or own them already. It's a lot more work, in short, so I understand why not everyone wants to go through that.
It's just a matter of personal preference. I don't think one is better than the other. There's no conspiracy, there's just the fact that if people are paying in-game bells or TBT for a villager they might not want to invest extra to fix stuff that could have been avoided.
You can't stop them from putting their furniture in Re-Tail
You can. You can put shells or fruit on Re-Tail marked 999,999 and they won't be able to sell or buy anything there.
Still doesn't stop them from mailing you crap, unless you want to get rid of your mailbox too for something as trivial as an item in a villager's home. Also, voiding out use of re-tail for the same reason is a steep cost. Personally I like villagers with pet sharks, adds curb appeal!
They rarely mail you things from their homes, Re-Tail, offering to sell you stuff when you visit and sending each other things through you is (in my experience) the most common way for them to change their furniture. There's no steep cost in voiding out use of Re-Tail unless you get a lot of money from selling things to villagers. You can still sell things directly to the shopkeeper (I can't remember her name).