I consider my self a hardcore player (spending some hours every day in the game, in the forum or some fb groups or with TE fb friends, keeping more than 10 teams with daily maintenance, even today which is some kind of a public holiday I 'm here) but those guys are real asso pro.
So, this guy has a team A' at lv 2, takes his 2* player and make him 6* (training only white skills), then sell him through nego to his team B' (3 levels above) as 3* .
Team B' makes him 3* > 6* (only white skills) and sell him again with nego to team C' (3 lvs above) with those skills.
Team C' which is his main team (the other are "factories"), keeps him only for asso games and no training - not to spoil the skills (or doing some training to keep this balance).