2. The teams buy the "wrong" players for co-ownership. In contrary to FM or even what FM-games might think, co-ownerships IRL in Italy aren't done to "test" young players, no way. It is too risky for both sides, as either one team pays for a player who doesn't become a good player, or the other team receives too little money for 50% of a promising player. And a seriosly working society doesn't like to do auctions after all.
So in real life these co-ownership transfers are almost every time done with 23-27 year old players, who didn't play that good for their team but still seem to have the potential. So the team sells 50% of the player and let's him try his luck in another team, however they still have the chance to call him back if he does well. It's like loaning out, only a bit more "I'm not sure he will be good, but I don't want to sell him completely now".
And the most important fact: IRL Co-owned players ALWAYS change teams. there are almost no co-ownership deals where the player remains at his club, however in FM it is almost every deal.