The "old" multi brand furniture eCom have probably been doing it the traditional way. Buying inventory and reselling it online as they would in a retail store.
But if you were to start a multi brand furniture ecom in 2020, I would recommend you start by reaching out to those established furniture stores and ask if they could dropship furnitures if you were to bring them new customers. You will be surprised most of them will be happy to work with you as long as you bring new customers.
On your end, it's a win win because you will get the same multi brand furniture for your store, without having to hold all the inventory.
Margin for furniture can go from 30%-300% - It really depend from where you are sourcing them and if you want to be a more affordable or luxurious furniture brand.
Hope that help!