Some context: Normally, players can trade items using either the «Market trade» or «Personal trade» features. «Market trade» lets you sell items through the Steam market system. «Personal trade’» is supposed to allow friends to trade items without any costs attached.
Recently, though, we’ve seen a few cases of players using the personal trade function to sell items using third party sites. This is essentially an abuse of the system. To prevent this, we’re temporarily turning off personal trades while we search for a better solution. Once we figure out a way to prevent abuse, the restriction will be lifted.
Thanks for reading, and we hope you understand!