So Pashmina decided it would be an awesome idea to put all of her rococo furniture in retail. Mairen doesn't think that's so awesome and would like to somehow return that silly goat's furniture back to her. I've heard that I could mail it back, but that only works sometimes. Also, if I try to mail it and she chooses not to display it in her house, is it going to get lost in some kind of "furniture void"? I imagine this fancy furniture is kind of expensive so I'd rather not risk that. Is my best bet to wait until she comes to me asking for furniture, then I slowly hand it back to her with each request?
It sometimes won't work if you mail it to them, but it won't be voided as long as you don't give them an overload of items. Each villager has their own inventory from which they'll try to sell, so at times you might find them trying to pawn off something they got from you in the mail.
I would suggest having them ask for furniture though, as that has a higher chance of making them put it out in their house. Rococo may be one of the more expensive regular sets around, but it's not crippling like the various sets Gracie sells. You should be able to easily earn back the money spent on rococo pieces.
I was trying to do this to one of my Marshals before. I got a Marshal from someone, and they lied about how original he was. His entire house was gutted, not a single sloppy furniture remained. Not even the sloppy table, and you know how hard that one is to get considering there are items on top of the table. Anyways, the sloppy series is expensive. I managed to get my hands on one full set, I wasn't going to leave it to chance. The mailing system is exactly just that, leaving it to chance.
I preferred to let him ask me for furniture instead. That way it was a for sure thing he was putting it in his house. I also sped up the process a little bit by cleaning up some of the clutter/junk that was left behind. Whenever he would ask me to come over, I'd always ask to buy/take an item.
Sometimes the furniture disappears if they don't put it on display, yes. You can always order the items from the catalogue again because they'll be added to it when you buy them from Re-Tail.
If you don't want to spend money, just send her one piece at a time. An easy way to make sure that she uses the furniture and displays it in her room is to mail it to her. You can send it shortly before 5 PM and then a few minutes after 5, enter her house and check if it's correctly placed. You can do something similar by getting her to trade or buy the items from you. Just empty your inventory or fill it with a bunch of useless items like flowers, plus one piece of furniture. Go visit her and she'll eventually offer to buy it or trade with you. After you agree, leave the house and go back in. With both methods, if you don't like the placement or if she's not using the items, you can reset and try again.
Which items does she already have? Sometimes when they're putting up furniture you've given them, they'll put it in empty spaces before replacing it (if they can). If there's something in the spot where you'd like something else to go, just send the other stuff first so she'll have to use the space she has for the furniture you're sending.
It also helps to talk to her often, because she might ask you to bring her furniture and that's another good opportunity.
In the future, if you want to keep villagers from selling their furniture on Re-Tail, just fill it with fruit or shells priced at 999,999. They won't be able to buy them or replace them with their own.
Buy the furniture to be sure it's in your catalog. It's not terribly expensive though. Use that reset trick that Laser Beams is recommending too. Don't give them any bugs or fish for now either, because those are too hard to shift.
One other suggestion is to always visit when invited over and talk to them until they offer to sell you a piece of furniture. Buy a piece, and then mail them a piece that will just fit the space you created. As long as their house isn't really empty, I've had a lot of luck with getting them to fill the new space with something I send them.