On October 28, 2019, Valve disabled the trading and marketable status of all newly purchased container keys to combat large-scale fraud and money laundering.
Because of their finite nature and historical use as a stable, cross-border trading currency, legacy keys command massive premiums compared to the standard $2.50 USD in-game equivalent. 🛒 Methods of Acquisition
Keys that were already sitting in user inventories prior to this update were exempt from the restriction. They remain fully tradable and marketable to this day.