Why do you want to sell an 18 y/o 5* FT player and replace him with.. ahem... some newer version of himself?
Buy three guys like this at the start of the season, feed them all you have, buy the next three at the start of the next season, feed them up... Four seasons, and you have a young FT-team. And also this is the time you need to replace those first three guys, because they're getting old.
It's very unefficient to spend your greens on old players. Train them while they're young, forget about 22+ y/o mastodons. You are wasting packs training them. Or do I miss something here?