Today, Valve released a blog, which said they are restricting players’ ability to trade and resell CS:GO container keys. According to Valve, the restriction applies only to newly purchased keys from now on.“ Pre-existing CS:GO container keys are unaffected–those keys can still be sold on the Steam Community Market and traded.”Valve said in the blog.

Valve also give the explanation on this restriction: "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."

You can read the full blog below:

Starting today, CS:GO container keys purchased in-game can no longer leave the purchasing account. That is, they cannot be sold on the Steam Community Market or traded. Pre-existing CS:GO container keys are unaffected–those keys can still be sold on the Steam Community Market and traded.

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.

For the vast majority of CS:GO users who buy keys to open containers, nothing changes; keys can still be purchased to open containers in their inventory. They simply can no longer be traded or transacted on the Steam Community Market.

Unfortunately this change will impact some legitimate users, but combating fraud is something we continue to prioritize across Steam and our products.

If you have feedback or concerns about this change feel free to email us at CSGOTeamFeedback [at] valvesoftware.com with the subject “Key Restriction”.