good luck trying to sell them. many free agent +32 YOs often cry themselves to sleep at night (no one buys them)
if you do renew their contract it might just extend it until the game decides they retire. lets say you resign a 31 YO and they end up retiring at 32 YO, itd be kind of a waste
so yeah, you should try to sell your players before they hit 30 and hope someone needs an emergency transfer