Try tp save your coins for the transfer market as this is the place where you will find rare or popular skilled Players that could end up being mainstays of your team. Although these players tend to go a little above market value it is worth regularly checking the transfer market as there is always the possibility of finding players that although not well known will have a high stat score and be right for the formation your team plays in.
What is a Free Transfer?
This is either a player that you released from your team, or has been randomly released by the CPU. The ratings for these Players tends to be low. To acquire a Free Transfer you need to go to the menu and tap the 'Transfers' button followed by 'Free Transfers', from there you will be able to sign anyone you want absolutely free. Note: Free Transfers don't count as actual transfers.