Steam In Game Items.

Anybody who is familiar with the Steam Community Market knows that there are two market categories:

1) In Game Items. If you navigate to the steam market and click on advanced search options, there are a list of games. the list of games has many games, including Steam(get to that later). If you click on the game, you can find many In game items that you can USE IN GAME.

2) Steam. If you click advanced search options and click steam, you can choose what steam game to search for, and it will shows steam items for that game(trading cards, emoticons, etc.)

The Popular Unity game, Rust, has an in game items market page, where you can purchase in game item skins. I would like to know how this is done? If anybody is familiar with this or can point me in the right direction, I would appreciate it!

Thanks in advance.

P.S. I am trying to do this in Unity 5.