Container keys, which you purchase with real money and unlock loot boxes, can no longer be resold. Basically, you can no longer sell them on the Steam Community Market.
Valve recently learned that they’re being used to launder money. On the Counter-Strike website the company wrote, “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.”
They acknowledged that some legitimate users may be affected, “combating fraud is something we continue to prioritize across Steam and our products”.
Valve also included a brief statement that “pre-existing CS:GO container keys are unaffected–those keys can still be sold on the Steam Community Market and traded.”