1) At level 2, you can get free transfer players with maximum market value of around $1.25M. Those free young players are really fast trainers. If you can get them, that's good, but avoid bidding wars. Patience is the key in winning free players
For DC/DL/DR/DMC, just train only defensive skills to the max. MC/ML/MR/AMC/ST/AML/AMR, training only attacking skills. After maxing out training either defensive or attacking skills, then you can train Mental attributes.
For those players, use them for 1-2 seasons and sell them to the market. The market at low level really loves the players concentrated in specific skill as I mentioned above.
This advice is to make money from buying and training free young players. But remember, be patient and back out of any bidding war.
2) Try to get young and good players (20-22 years old + 5 stars) with 3 playing roles, the higher market value of them the better. Keep them in regular squad and use them for 3-4 seasons, depending on future circumstances. Later on, if you don't want them, you will always be able to resell them at maximum value.
3) if you buy high quality, old players just for temporary squad, you should get the one with special abilities that match their roles. You can resell them more easily than those without matching SA.