unfortunately the trading argument has been made many many times and it just is never going to work.
I have seen before, and im not sure if this is against the rules or not, but two teams whom come to an agreement to 'trade players', put them both on the transfer market at the same time, for above market rates.
Then both players buy the other teams player. money lost, money received, you lose a bit on the sales commission but nothing too drastic.
in my opinon, doing it this way is a bit tricky, but simulates trading.