no use to renew a 2 or 3* player..u will find a better or same next season and save money .
For 4* it depends in my opinion.I have done both things (renew and sell) in 4 seasons .
I cant tell which scenario is better.

In first 2 seasons i was trying to understand the game so i renew some players,also my server was so crapy that i thought i couldnt find better player next season.
In 3-4 season my server changed a bit i can find at least 4* players,so no use to renew my 4* .

Generally in my opinion no use to renew 3* players(totally crap next season) and it depends to the server for the 4(that will drop to 3) next season.