I hate this misconception we sell our best players/young players. The only two significant you players we've sold in the past DECADE are Rooney and Rodwell. IMO Rodwell wasn't even that good and think the 12m we got for him was excellent business.
So I'd like to know who else he's referring to? He seems to have already forgot what happened at his time at everton.
I think maybe you need to go further back, obviously we've sold the likes of Ball, Dunne and Jeffers in the past when they where touted as the next big thing. That's 5 in twenty years. During the same time we've only managed to keep 2 players who've come through the academy and had top careers in the premiership, so generally we have sold our top young prospects.
Hopefully that's going to change though, but he's right when clubs have come in for our good young players we've sold them on.