I think you should allow managers to post articles in the press room so that they can let everyone know what they are thinking. For example, they could say that x player is for sale for x amount of money.
Also, for transfers there should be a feature where u could do swap deals.
I have heard some managers say that before. Not a lot say both though. . To answer your question well I agree with making manual articles available in the Press room. Another Online game I play has that (but I am not sure OSM allows the mention of other games on their forum ). But for the players, if another manager wants your player they could either buy him on the list or make an offer.
As for the Trading of players, there could be cheating; where I swap a player with 40mil value for one with 3 mil value.
Again that other online game has that. . There you cant buy a player for anything less than his price. But you can make a cash + player deal of equal value. OSM could try that.