Everybody wants their own players to succeed. You hope every vet has a career year, and every prospect is blue chip. It's only natural to over value your own players. That can make trading difficult.

I once posted an offer for an average prospect. Good minor league bat, but no defence. Because he is slow he'll never have a huge PB value, no matter how good he hits. He was a third round rookie pick, fourth in most leagues. The counter offer was ridiculous. He asked for a young productive stud PB7+ and I couldn't believe it. Completely unproven, there is no way this guy could ever reach the value of my guy, if he makes it at all. It was obvious that this manager has convinced himself that his rookie is going to be spectacular, despite what everbody else thinks. So there is no point even trying to convince him otherwise. I just rejected his counter offer without comment.

A lot of managers over value their own players. There's not much anybody can do about it.