I think it's best to have a healthy balance of training players to keep long-term and training to sell for a profit. Some people prefer to train players for their team and keep them until they turn 33+ or their whole career. Others like to buy players, train a little, then flip them for a profit. For a user starting out on BB, I recommend training for your team a little over training for profit. But it's up to you to decide what you want to do.