When you collect Mush Furniture, don't sell it to Tom Nook. It may seem a lot of Bells at the time, but you can get up to 5,000 Bells if you don't. Wait until the Flea Market, held on the first Saturday of every month, and wait until a friend comes to shop at your house. Before they come, put out all of your Mush Furniture. They will randomly go over to an item of furniture, and if you are willing to part with that furniture, they will ask you for an asking price. This could be any amount, but if it is too high they will ask you to drop the price. It is up to you, and if you choose to do so, ask for around 5,000 Bells. If they say that this is too much, go down 100 Bells at a time until they accept your offer. You won't be able to buy it from Nook's store, but you can get more in an Acorn Festival or get another housemate to collect Acorns from the Acorn Festival.