You just don't get it.
It's not leaving per se, it's HOW the player leaves.
As long as Barkley doesn't - say nothing all summer whilst speculation rages, then just before the season starts, tells the manager he's not in the right frame of mind to play for the first team. Then hands in a transfer request 3 days before the window closes to force through a deal, allowing no time for any of his fee to be spent. Then after moving, repeatedly kisses his new club's badge - on occasions after scoring against Everton, and instead of being a pro, taunts the fans by cupping his ear, etc. Also, subsequently writes lies about his former manager in a book, which he later admits are lies and settles out of court. Oh yeah, and expresses no gratitude to his former club for several years, for all the time they spent developing and nurturing him.
As long as Barkley does none of that, he should be just fine.
Look how Arteta left. There might be the odd one or two who have issues with him, but when we played Arsenal away earlier this season, his name was applauded by travelling blues when it was read out by the announcer before the game. He'll also be applauded when he returns to Goodison soon.
There are 'proper' ways of leaving Everton that ensure you aren't booed. Most of us don't boo or 'hate' someone for no reason.