I think you spend wisely if your players perform well. If you sell a low 7-8 performer that is always good morale and quickly rested, and buy a low performing 5 star that you constantly have to boost, then maybe not such good trade-off. I have a really good MC that constantly misses on his shot in the comments. I will boost his attack but more then likely I will be selling him. I also have a 3 star forward that always scores. I am trying to figure out why but he is gold boot for three seasons. I will keep him no matter what.
You must stop using the red and blue arrows for everyone and no one seems to be using man on man. I only use man on man when my defense players numbers are larger then the opposing forwards and I line them up to match with the blue arrows. The arrows put your players in those positions and the game will lower their fitness numbers. If you insist on using arrows, make sure you boost the fitness numbers for those players when assigning skills.