VMK was an online "virtual world" game Disney made that sadly closed back in 2008. It was based off the parks... you could play minigames (like Pirates of the Caribbean Ship Battle and Haunted Mansion Ghost Chasers) to get credits that you could buy Disney-themed items with. These items could be used to decorate your avatar or room... a lot of users even made games with these items. You could trade most of the items, and many of them became rare after they were retired. Many users wanted to collect all of the items, so trading was a central dynamic in the game.