From today, Counter-Strike: Global Offensive's loot box keys can no longer be resold on Steam's marketplace. Valve are putting a lock on their keys because of the global gangs of evil-doers who were using the keys to launder money. From now on, any key bought will be bound to the purchaser's account.
If you bought keys for the purpose of unlocking a container to get at the sweet loot inside, this won't affect you much. You can pretty much carry on being disappointed by the skins you unlock. But those who were trading keys to cleanse their ill-gotten gains are now locked out from doing so.
"Why make this change? In the past, most key trades we observed were between legitimate customers. However, worldwide fraud networks have recently shifted to using CS:GO keys to liquidate their gains. At this point, nearly all key purchases that end up being traded or sold on the marketplace are believed to be fraud-sourced. As a result we have decided that newly purchased keys will not be tradeable or marketable."
Keys bought before this change can still be sold on the market.
Reading into this on Reddit, it looks like the keys were used because they had a consistent price across all of Steam, so traders, both legitimate and heinous, used them as a base currency. It looks like with CS:GO's keys no longer in play, they expect people will switch over the Dota 2's Arcanas or TF2 keys to carry on trading. Still, they do seem to understand that it was as necessary, and even expected, move.