Milan absolutely sells player generally low, I can’t believe that’s even being debated. There are far smaller clubs than Milan that ask for crazy high prices and refuse to budge on them. And I’m not talking English or Portuguese clubs either, look at Frankfurt demanding a crazy fee for Ekitike. The whole “player has an agreement with club X” is so overblown too, most players don’t want to sit on a bench for a year and if there’s any time on their deal, you can definitely hold that over them.
I think it has to do with we are bad at selling our deadweight players, so they sell anyone they can for a big sum to have at least some cash flow. Also Milan hasn’t usually been a club like Juve that’s ruthless with players, if an opportunity comes we generally try to let them take it. You can argue individual transfers all day but in a general sense it’s very obviously true Milan doesn’t really hold out for absolute max selling value.