Another rule: upgrade your stadium on every first possible moment! Level 2 is little bit too few after 3 months. It's better to upgrade TC to level 10 on bonus period and then start to upgrade YC, meanwhile taking 3 loans(better than your own players) and buying some cheap youngsters with talent over 4(something like Tauber and Frye) who would add something to your squad. After some seasons you can sell them to bots or bank with some profit and replace with better ones, gradually improving your team. I did so with my Austrian club, earned promotion after some L2 seasons, avoided relegation and had some top 4 finishes with euro spot and won couple of trophies too.
At some level stadium will be your main income, but you need it some more levels for that.
That overload of those players you indicated comes because most of teams have YC10, so market is oversaturated of such players, most of them are on market until asking price is reduced to bank value and then it's better financially to sell him to bank than reduce price more to attract buyers.