Introduced in the Temperature of Love Update (Patch 2.1.17), mystery boxes bring a whole new level of selling items away!
Pack any items you want into a mystery box, name it, add a short description and list it for any price you want! Wait for someone to purchase it, and the surprise is theirs!
Mystery boxes are part of the Black Market, and are not physical items that will be inside your inventory once purchased. Upon purchase of a mystery box, you can check the items you have received immediately.
There are no regulations to this system, everything you buy from here is a mystery and there is no guarantee.