If an NBA team trades a player and that team waives him, the trading team is not allowed to sign that player. So, if the Bulls were to buy out Anfernee Simons after the trade deadline, the Celtics would not be able to pick him back up.
However, if a player were to be traded a second time, then bought out, the original trading team would then be able to sign that player as a free agent. In other words, if the Bulls flip Simons again before the Thursday deadline, and the team that acquires him buys him out, the Celtics would be able to sign Ant back to the roster.