It is increasingly difficult to sell players at the higher levels as many managers ran into financial problems that they couldn't afford new players.

You shouldn't ran into negative balance, the bank will claim interest and it is very difficult to come out fine over heavy debt. Even if you recover from it you still lose out.

If you really like a high quality team without paying for tokens, you may try to focus on money making for a few seasons by keeping your team quality lower and training youngsters for the sole purpose of reselling. If done properly I'd say one can reach max cash (2.1b) after 2~4 seasons at your level.

Of course, in this game you can always pull out your wallet to make up for your bad decisions