The trade block, or trading block, is the informal list of players a team is willing to trade. "Putting a player on the block" tells the league the player is available.
In HoopSim the trade block is a real feature. You list a player with up to four tags for what you want back (picks, young talent, expiring money, or a position) and a one-line note; rivals see only the players you list. You can list three players per calendar month, and the block closes when trades do. Listing gets noticed: a card goes up in League chat, GMs thin at that position get a push, and a star listing can make the wire. It costs something too: being shopped hurts the player's morale, more for a star and more the longer they sit there, and each listing dents team chemistry a little. The sting fades over about two weeks after you take them off. AI teams list players too.
Rulebook: The trade block. Related: trade deadline, rebuild.