As far as I have been able to tell this breaks down in to 2 additional areas. Spaces (rooms) and inventory items.
I do not have experience with the selling spaces. There are SW adn player spaces for sale under "Places" "For Sale" It is my understanding that any property purchsed there comes completely unfurnished. (Yes, regardless of the photo)
I do sell items in the original room. When Sio Smallworlds had her pirate related mission she had drinks on a bar. I had also seen this in a few players' room. One day I was decorating and decided I'd put a few food items that I had in my room "on sale". They sold. Now I sell food and some magic tricks.
To sell an item that is in your room.
Click the item
Click Sell Item
A box "Sell for:" displays the in store price filled in. The owner selects their price from 1 credit on up.
Select a price, and click "Sell"
An "Item For Sale Message" appears in the top center of the screen.
I generally charge an extra 2-3 credits on food/magic items. If I am in the game when an items sells, I receive a "ding sound" and a pop up in the top center of my screen. Either way the tokens earned show in "My Account".
(There is also a stop selling button.)
It's funny, even when I whisper hte items are cheaper in _____Store, some still buy.
I have not seen any "commision" like on Gaia.