A few things really, when you press the accept button on transfers (with managed clubs) then we think it's a bit shady to then change your mind (as the deal will be announced as accepted and you often get players going "you could have done better than that", "omg you're getting ripped off" etc etc).
Also we have a Financial Fair Play system that means that we all have a squad cap. Without checking, yours will be around 52 players. So make sure you have 50 or less players (this is done to prevent the top clubs with loads of money having 100+ player squads). It's balanced so the bigger clubs have less space while the small ones have more space.