Salaries dont work either, a 50k center is so much "cheaper" than a 50k guard at the moment, so you could only trade very similar players on the same position and who would like to do that?
and even then you will get trouble with real Sf's^^
And also betwen a league you could cheat, or deal - i could imagine that lot teams will build alliances like if you made the Po i rent you my star player because i like you mroe then your opponent etc.