Videogamer has posted an interesting article regarding an in-game auction house similiar to Diablo3 where players can share content freely. As of yet they are not saying whether users will be able to sell said content (and I certainly hope not), but I'm sure more will come to light as development progresses.
Read the article here (http://www.videogamer.com/pc/the_sims_4/news/maxis_developing_in-game_platform_for_the_sims_4_content_sharing.html).