Like the trade players idea, but it might be hard to control. Perhaps it could be held with the possibily of vetoeing a trade, like in yahoo fantasy nba. Would it have to be 1 on 1 trades? Could it be 3 for 1? Would it have any tax?
The "package" selling is a bad idea, don't like it at all.
What could also work is the loan idea, borrowing players for a certain amount of money. It could even give some economical reward if the player was trained.
Perhaps this might prove to hard to be brought to BB today, who knows in the future.