There used to be very few trades in the NFL. But teams have become more willing to do so in recent years, acquiring veteran players either in training camp or once the season starts.
They happen more often, obviously, in the other major sports leagues, but are something more of a new phenomena in football.
The Steelers have been active on that market, making several moves in recent years to acquire veteran players, most recently doing so with to pick up linebacker Joe Schobert and cornerback Ahkello Witherspoon in training camp in the past month.