Counter-Strike: Global Offensive has launched the "Arms Deal" update today, creating a player marketplace for in-game cosmetic tweaks to weapons. The update also adds weapon cases, special timed drop lock-boxes that can only be opened by buying or trading a key.
Starting today, you can buy, sell, and trade your own decorated weapons, or trade keys for the weapon cases. It adds more than 100 weapon finishes to help you customize for trades, but Valve stresses in a FAQ that they're only cosmetic changes. It also mentions that they're working on workshop support, so you can make your own finishes. You can sell unused weapons in the Steam Marketplace.
The Marketplace is also selling a special eSports Weapons Case, which puts money toward the coffers of prize purses at competitive events. For classic Counter-Strike players, the update adds two classic weapons: the M4A1-S and the USP-S.