Before you go selling to make room for academy players bear in mind you're going to get a 2 star player for 0 tokens and a 4 star player who is going to cost you 8-12 tokens.
2 star player is good to either sell for extra money or hang on to for training bonuses and draw manipulation.
4 star player you'll have to think carefully about. Do you want to spend the rest packs to train him up to be a 100/120% player?